A similar thing happened to me recently
I ended up replacing my 8800gts with a gtx275 and everything seems fine. Except for L4D, which BSODs or the video drivers stop responding.
Theres a tweak floating around.
1. winkey + r
2. msconfig
3. Boot tab
4. Advanced Options...
5. Check Number of processors
6. Select the optimal amount of processors
I usually play on the Rogue Squadron server at 8.9.15.234:27015. It's on 24/7 Gold Rush and doesn't have those annoying plugins that all those other servers have.
I had to call in, and get someone to help reactivate it (yeah it was a real person). In my case i was actually just replacing my motherboard and keeping everything else the same. It works now!
Hot air rises. So if you have a fan on top, I'd imagine it would blow up and out more effectively. I haven't examined the air dynamics, but this is just something I've assumed..