I have been having this problem since the beginning of the game and really have been frustrated with it. But this was sent to me, and it does seem to work (I have only tested it for a few hours). The trick is do not use the quick save (F11) key! Always save your game by using the main menu (ESC - SAVE). I was a little skeptical about this at first, but it does appear to help. Since I've been doing this I have not experienced a single crash.
UPDATE: It has now been reported that it may be the saving over and over again to the same file that is causing some of the problems; try saving to a different save slot and see if that helps your problem.
NOTE: After spending a few more hours with it, I have to say that it seems to help reduce the number of crashes, but they still happen. I can't say for sure that this is a cause or just luck (bad luck?)
It also appears that many people who are having this problems have either TNT2 or GeForce chips. It has been suggested that underclocking the card by about 5 MHz will help reduce the lockups also. Make sure you have the latest reference drivers from nVidia, you can get them here.
BIG UPDATE: Going on the idea that underclocking the card will help the lockup problems, and the thought that my machine has been exeriencing heat problems (the game would lock up much quicker if I had the machine running for a long time before starting the game). Inside my case I have a fan that is moving air at 80cf/min! That is some major air flow, but the problem is that it doesn't help if you are moving hot air. So this weekend I pulled out my reciprocating (sp?) saw and cut a 4" x 4" hole in the side of the case (covered with a screen to keep baby hands out). So far I have had ZERO lockups and this is after some major playing, the machine was on all weekend, and our tempurature was close to 85! This looks like the solution; maybe the nVidia chips are running just a little too hot. (if someone wants to send me a GeForce I'll test it out for them to see if this would help).