Originally posted by Nubius
If you see your +12v rail hitting down at like 11.6v or lower just from running prime95, then you can pretty much assume during heavy gaming when RAM, CPU, and the video card are pulling lots of juice that it's going below that 11.6 and causing the reboots.
I think this is the problem here. The PSU sucks and under heavy load regardless of the wattage of the PSU, the +12v Rail is probably sagging to a large degree. Once the voltage drops too much, the system freezes or restarts.
19a & 400watts should be fine for that PC. But with an unstable +12v RAIL... This problem is normal.