What's causing distortion at musical climax?

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.

Thanks for all the great responses!
Tracking force could be an issue, since I've maxed out the adjustment on my tone arm and I can only get 1.5 grams. The Ortofon recommends 1.5-2g for that cartridge. What's the best way to add 0.5g to the cartridge end of the tonearm?
I'll be borrowing a friend's tube preamp, so we'll see soon whether the Radio Shack box is part of the problem.
I'm confident that that the amplifier and speakers are not a problem - CDs sound just fine, and the amp doesn't have a phono input.
Thanks, guys!
Mika

I just got a Music Hall MMF-5.1 with the Goldring 1012GX MM cart. It recommends 1.5-2.5 grams I believe, but the Music Hall manual suggests that you set it at 1.7. I also am getting distortion at some points, especially when getting near the end of a side towards the middle of the disc. Could this be corrected by increasing the tracking force? I might play around with it once my cart is fully run in.