Help! Power question

It sounds like the power going to the audio rig is not on a separate circuit.  Contact an electrician to confirm your house power configuration and service amperage. If the house service amperage is insufficient, then maybe your rig is suffering a transient brown out condition. I’m not sure a surge protector would help in that case.

Mlapenta, you are experiencing a massive dip in your current from the panel....line conditioners only massage the voltage.  When the ac kicks it has a high instantaneous current draw to get it going...the ac is 240volt so it is fed by both phases of your panel, therefore you can not escape it entirely, but you may be able to mitigate the effect to where it no longer causes your gear to freeze.  Many things could potentially lead to the prob, but the most likely is you have At least 1 of several things (or a combimation of several things) occurring that would cause an end effect such as yours:
- You have a 100 amp panel and the a/c initial draw is a very hi percentage of that, causing a dip housewide that effects your equipment
- your panel was out of empty breaker locations, so the ac installer used the spot for your audio line, and then just threw them on the same breaker....check inside your panel for 2 wires going into one  of the ac breakers (ac breaker will be a 2 pole which occupies 2 slots in the panel....the ac should have 2 wires going in....1 per screw clamp....if there is 3 wires going in, with 2 under any one screw clamp, then that is what they did....)  also be sure to check the breaker for the air handler which could have the same scenario going on.
- your panel is so old that the dielectric is effected, thus minimizing it's amperage carrying capacity. Is there rust in there? 
- loose connections on your equipment or thru the line will increase the impact of the drain...
    - are your cords secure in the outlet and at the rear of your equipment? ...
    - is the outlet secured firmly to the wall?
    - Is the outlet damaged? 
    - Are the splices (if there are any) within the outlet box solidly connected by being  twisted together under the wire nut, or are they just put next together with a wire nut twisted on top of them
-  is the audio equipment home run at the panel secured properly to the breaker, the neutral bar and the ground bar?

Make sure your power is OFF before you start messing with teceptacles, the panel, or any exposed wiring! 

Hope this helps & best wishes!

PS. ... IF you are not familiar with electric wiring and experienced at handling it, an electrician is really the way you should go, with 1 word of caution.... if you are not familiar with the electricians reputation, ensure he checks everything, rather than just walk up to the panel and say you need a new one. ..   big job... big $... he likely wouldn't lie, but that doesn't mean it is the simplest solution to the problem you want fixed at this moment when securing connections might do the trick for $100 instead of a thousand.

The house may not have enough amperage available on the line entering the house to add a big air conditioner to the existing line?Particularly old houses just never had the idea of 100 amps being used..A response from a Googled question about lights dimming (which is similar)"" Start by checking your voltage as close to the source as possible (like at the meter base) when the A/C or heat cuts on. If you have an excessive voltage drop here, you may need to get your utility involved to check supply voltage, transformer loading, supply connections, service wire size. If the problem is beyond the meter, check your voltage drops moving towards the lights and A/C until you find the problem (small wire size or bad connections are most likely). ""

PPS...  I mistook your "surge protector" for "power conditioner"... that in mind, a surge protector only protects your equipment for HI power spikes by cutting them off....it can not effect low power dips.

TY all, very informative   ps: the new air is a 2.5 ton unit and "modern" the old unit was 25+ years old 3 ton and did not cause this problem
interesting
thanks again

That is interesting now that I understand it was a replacement and not a first time install. Hmmm ... it has to be the new unit simply has a much higher initial current draw to get going than the old one did.  Yet, everything we all stated still applies, except my comment that they may have used your breaker position from the audio line, since the positions from the old ac units breakers would be available.

Most older AC units had hard start devices built in some newer models don't have them if yous doesn't it might be why the old didn't cause a problem and the new one does. Do you notice if any lights flicker or dim when the AC kicks on ?

do not notice effect on lights, will observe closer next time.  thanks all

Call an electrician. There is probably a loose lug connection in the meter pan or main panel on one of the phases.

Did a licensed electrician do the connection on your new ac unit? If not, you need to have everything checked. Refrigeration people do not have to be licensed electricians in my experience. Who knows what was diddled on the install. 

As djones51 mentioned, having a "hard start device" put on the A/C unit should be a first step in my opinion.  I think of it more as an "easy start" because it reduces the surge needed to start the A/C unit.

You could also put a "Heat Recovery Unit" on, which costs about $500 - $750 and will provide heat for water in the water heater.  I had one here in Florida and could turn off the water heater from May through October.  It might be considered unrelated, but the less the demand on the panel, the better whatever surge can be handled.  Also, this increases the efficiency of the A/C unit by about 0.5 SEER.

Finally, and this might not be approved of by the audiophile group, you could add an APC Smart-UPS 750 or similar sine-wave unit.  I don't really understand the technical arguments against this, as I think as long as the UPS can provide the power it should be fine.  Now, a little APC 350 might not be the best. I do think it should be a "pure sine" UPS, not a "stepped" approximation.  (I was able to run an "energy star" refrigerator on an APC-SmartUPS 750, but the non-"energy star" refrigerator wouldn't work because it created too much surge on startup, so the UPS shut down.)

The "surge protector" is just for "protecting" and not for "preventing."

Perhaps a Sola constant voltage transformer would solve this issue. We used to use these for isolating test equipment. I have seen them on ebay for not much. Like under $200 for a good sized unit. 

+1 gs5556 (He would know)!

I agree with everyone saying get an electrician over to sort it out for you. But in the meantime you could try a line conditioner like this to allow you to work around the issue: 
https://tripplite.com/1800w-120v-power-conditioner-automatic-voltage-regulation-avr-ac-surge-protection-6-outlets~LC1800

(get the right wattage size for your needs there are multiple models)

I use one myself and while I don't have any issues I do hear the relays in the line conditioner click on and off every time my AC starts and the LC "boosts" the power.

thanks all again
re: the tripplite mentioned, many reviews says it has a rather loud "hum"

Throwing more devices on the audio rig circuit likely won’t help things.