Windows XP use to have an option somewhere to disable autoreboot on a bluescreen, not sure if W7 has this same option or not but that might be worth looking into as well. though since nothing is showing in the even viewer then that probably wont help any.
My gut feeling would be a point heat issue over some componant. Mostlikely something that wont show on a standard temp monitor. If you dont see any issues with a temp monitor progam running try altering the box configuration to change the air flow and see if that does anything.