Almost two weeks ago, I wrote that I suspected that a memory issue was causing my MacBook Pro stability issues. I bought a new 2Gb memory stick ($70) and haven't a single problem with my MBP freezing. Maybe the old memory was bad, maybe it just wasn't working well with the MBP (memory can be finicky), or maybe it was running hot and causing a thermal problem. I don't know. But for now, replacing a single memory stick seems to have solved the problem.