Valve have said that there's a problem with certain nVidia cards whereby they stutter, then performance dies, then the app crashes completely. It's apparently something Valve are doing that's cacking up the texture management code in the drivers, but they're having huge trouble isolating it you can have two seemingly identical machines, one of which has a problem, and one of which doesn't.
For the record, I don't have a nVidia card (at least, not in my main PC), and HL2's been perfectly stable for me. Stuttery as hell initially, but stable.
Also, there is a v1.1 patch for Doom 3 which was released as a beta v1.05 patch - I believe it sorts out a few multiplayer issues.