After the memory test, POST next looks for the IDE connnections - HHD, CD, DVD. How far along this process do you get? I suspect that, as your PC warms up, something that was lose tightens up. Check all connections to your IDE drives - power in particular.
Also suggest that you reset the BIOS not to do the boot virus check to see if that solves the problem. If it does, check to see if you need to update your BIOS.