It might be overheating. The original thermal paste between the CPU and heatsink has almost certainly dried out, if it hasn't been renewed. Also check that the heatsink clip is the right way round - it's not symmetrical, and the lowest point has to be over the CPU die. While you're checking that, might as well redo the thermal paste. Searches here will reveal how to do it.

If you suspect the power supply, you can replace it with an ATX supply by changing the position of a jumper on the motherboard from "Mac" to "ATX"