I've made a new catalog, and so far, with just a few images added, have had no freezes. I also purged the camera raw cache, and went back to the old, troublesome catalog to see if there was any improvement there. While not completely free of freezes, the long image load times have improved, and the tool problems I was having (clone and heal tools were non-functional) are fixed. I will continue to re-import saved images from disk to the new catalog, but before doing that, I think I will see how it goes for a bit longer with the original catalog.
Thanks again for your help, and to Francois as well.
I have had the same crashing problems that left several dozen adjustments undone. I was in the middle of changing my drives around, so as soon as I got all my images on the backup computer about 20,000 (almost 300GB), I took care of the cache and simply copied them to the library on a reformatted drive. I then opened a new catalog in LR3 and added all the files in the library. I was surprised it only took about fifteen minutes to import them and then I left it to do the previews. So far everything works flawlessly now with the new catalog in LR3--no mo crashes.