Thank you all for your replies thus far. I've placed several sheets of negatives under a couple of heavy books for the last two days, and that has seemed to help tremendously. I do think much of the problem has to do with the lack of humidity in the air (we have a woodstove, and it's been fairly cool lately so the air is relatively dry) hence the film dries quickly. Additionally, that Plus X was expired some time ago and had therefore been sitting rolled-up in a cassette for quite some time. I can't explain the new Tri-X curl except for the dry air. I'm going to try weighting the drying film more to see how much that will help.