Most likely heat related. It might be the CPU heatsink paste having dried out. It would be worth booting into BIOS and looking at temperature levels recorded or find of the the system monitoring applications. It could be other critical components or solder joints that also fail after reaching temperature over a period of time.
(Does the system run for a few hours again after rebooting or does it fail quickly - if the latter it would definitely be a heat issue).
The most sensible way to isolate it is to swap components - but this might be costly for you - though it is a fairly old system (5 years??) from what I see from the models quoted.