Analog Help Please

Well I'm certainly no authority here, and will try "humbly" suggesting something. I certainly do recognize the peerless quality of both Arm/Cartridge you are using, and gather that you have much good experience with this high level of quality components.

What I thought of, was something more likely amiss with your Phono Stage, or Step Up, or Pre-Amp? In that perhaps it's just coincidental to be seeming like it's the Turntable-Cartridge that's causing this effect?
(Like there's something oscillating?, a bum Tube?, etc)

Can you somehow perhaps borrow-loan another Phono Stage to try?

Hopefully Doug D, and others here who have forgotten more than I know, can chime in, and better pinpoint this malady? Mark

Systems can be finicky in proportion to their refinement. Here are a few thoughts out loud....

I don't know anything about the Atmos, but if the cartridge is that new, I'd get it past at least 60 hours before worrying too much. Cartridges can act inconsistently from day to day during their break-in period. E-mail Mehran (?) and ask him what to expect for break-in.

If it is at a "particular place in the lp" - could the record have a small warp or bubble in that part of the groove?

Has there been any significant temperature or humidity change during the day to evening period where it happenened then didn't then did? I'm not sure if temperature/humidity can affect your cartridges compliance, but I suppose it is possible.

I doubt that compliance is the issue. The Triplanar and ZYX should be a good match. But since your symptoms sound like a similar experience I had , I'll relay it. Several years back I had mistracking issues with a Grado Sonata on a Premier MMT tonearm. Always on the first band of a side closer to the lead-in. I'd get mistracking on most any Telarc I played. It was an issue of stylus/cantilever compliance - Grado's were undamped (may still be for all I know) - which got sorted out as a cartridge/tonearm matching issue. When its not quite right, resonance can occur that causes what you describe as a 'flutter'. Increasing the effective mass of the tonearm or introducing some form of dampening both helped my situation somewhat, but not completely. You can change the effective mas by adding a gram of modeling clay to the headshell (and adjusting VTF) to see where that takes things. Dampening can be added by using some silicone (?) in the arm's dampening trough. You might observe the stylus/cantilever closely when the problem occurs to see if you notice any unusual 'wiggling' motion before and after experiments you may try. Again - unlikely that its compliance, but those are my thoughts.

The Tri-Planar arm has an adjustment for where the anti-skate starts to take effect. There is a small mono-filament line which is connected to the anti skate arm. This line can be adjusted so the anti skate takes effect as soon as the arm is lowered. If out of adjustment it could cause your problem. Start with 0 anti skate and when the arm is lowered check for slack in the mono line. If adjusted correctly there should be no slack in the line when the arm is first lowered. There is a small set screw which allows tension adjustment of the mono line. It is a bit tricky but not difficult.
Many Tri planar users have removed the dampening trough altogether. I have never needed additional dampening with my Tri planar Universe combination.
You have a great match between the ZYX and the Tri planar.

Yes, Mehran, the dealer for the Cart may be of help.
Only other thing that comes to my mind, is a possible slight bind, or some slight resistance in the Triplanar's bearings? How hard would this be to check?

After reading Doug's "tips and tricks" on this Arm, it sounds quite sophisticated, and complicated as well. Odd though how you say it's noticeable on a particular spot of the LP as well?

It's hard for me to suggest you playing your most treasured LPs to see if this problem exists as well with them, as if there is some mechanical issue (Stylus-Tonearm) I sure wouldn't want to see you damage this vinyl in a "sacrifice" test. Mark