What's causing distortion at musical climax?

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.

With the phenomenon you describe, worn used records seem to be the biggest culprit for me--some of my favorites among them. My assumption has been that years of mistracking in somebody else's hands wore the distortion into the groove. If I'm wrong about that, I'd love to know more...

Really, I just want to give a _tiny_ shout out to the Radio Shack pre: if it's the silver box powered by a 9v, Ultra High Fidelity magazine picked it as a budget favorite a few years back. It's discontinued now, but it's what I picked up when I decided to dip a toe into vinyl, and it did little wrong--especially at $20. I'd look elsewhere before blaming it.

I actually think that worn records may be the culprit here as Ablang described. I have a very old Living Stereo LP of Rachmaninoff's Piano Concerto No.2 that I picked up used for $1. It's a shame because it's a beautiful performance, and the quality of the recording is astounding for the time in which it was made. Unfortunately, it's just plain old worn out. Only 1 or 2 of my other LPs give me this problem of distortion, so to me it wouldn't make sense that the cause is anything else. The distortion only occurs during the loud passages as well.