Not knowing your specs, but your Vidcard would likely be the first to overheat and cause problems. I used some whatever visiontek card I got from BB and would crash like maniac when I ran 5 on my comp. Replaced it with a 8800GT and its like smooth sailing. You said that you got no problems with 1, so your video card would be high on my list of problems.

Also random resets can be due to a power supply either going bad or being inefficient.