I've been playing FTB (Mindcrack edition, single player mode) for a few weeks, and recently after updating to 8.1.1 I've noticed that the game has become remarkably unstable. I've tried reinstalling it, tried new worlds in Survival mode and Creative. I've tried with and without Optifine, but it doesn't matter. I've given it about as much memory I feel safe giving it (4.25Gb on my 8Gb system). I logged a ticket with the support team, but I also wanted to see if anyone else saw this problem. I searched the forums and the Internet in general, and can't seem to figure out what's going on. The only thing I can identify as a potential root cause is that this happened immediately after the upgrade to 8.1.1 of Mindcrack. Before then, it worked fine.
After about 20 minutes of playing the game, it crashes. It crashes hard. One time it crashed so badly, I was forced to re-install and reboot, losing my save game data (I had a backup, fortunately). This crash is very reliable, and for the life of me I can't see any reason why it does this. It worked just fine before. I don't have stability issues with any other application on my laptop. Vanilla minecraft works great. It's just this modpack that breaks.
My OS is MacOS X 10.7.5, and my Java version is 1.6.