What's causing distortion at musical climax?

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.

You left speakers off the list - they may not be able to handle the power or the drivers have reached their excursion limits.

You also do not make it clear if it's limited to vinyl only.

I think that the first thing to check is tracking force. You probably should start at the upper end of the manufacturer's range and then work down until you hear distortion (=mistracking) on heavily modulated areas of the LP, then go back up a little bit.

Before doing any of the above suggestions (which are all good ones that I completely agree with) you need to make sure your stylus is clean. A gummed up stylus will mistrack on dynamic passages that it could handle if it were clean.

Search for "Magic Eraser" if you don't know how to clean a stylus cheaply and effectively. Even if this isn't the cause of your mistracking problem, you may be surprised by how much better the music sounds after a good stylus cleaning.

After that, as others have said:

- try increasing VTF to near the maximum for your cartridge; use a reliable stylus force gauge, not the dial on your tonearm

- get a better phono stage; it's a highly likely contributor to your problem

- check the gain structure of your entire system; make sure no component is being overloaded

All the above is assuming these bursts of distortion are audible on both channels. If they're happening on just one channel you probably need to adjust anti-skating.