What's causing distortion at musical climax?

The problem is overload of the signal strength.
My first guess is that your cart is too high an output for the phono pre..
(the RadioShack phono pre should be going into a line level input in your preamp, not a "phono" input!!!!)
Get a cheap used 'audiophile pre' here for $100.
Your amp may be the problem (unless it never happens with CDs)

Aaahhhh....the old '' distortion interruptus '' I hate when that happens.

Another thought is that your amp has reached its limit (power wise), and is sending audible distortion to the speakers (i.e. clipping).

A friend of mine did this (by accident of course), when we were auditioning amplifiers for his Avalon Eidolons. He likes to listen to his classical music at realistic sound levels (in the front row of course!), and while comparing the Manley Neoclassic 250 to the VTl MB-450, he actually managed to get the Manley to clip. (Yeah, Yeah, I know, we were listening way too loud).

Just a thought.

Good Luck in finding the cause (and more importantly, the cure!) to your problem.

Yeah first response, Radio Shack goes to the wooden shack in back, or your garage! I really don't know your model or the sound so just kidding, but realistically I would start there.

As pointed out above, the two most likely candidates are mistracking (either cartridge/stylus or arm...or both) and overloading your phono pre. If a friend has a different phono pre they could loan you...try it...that should give you the answer.