Either that or your CPU is getting over loaded, and with you increasing the voltages it is useable.
I think that you need extra cooling though. You say it's pretty old, it's a good single core PC. During the time you've had it, the CPU or motherboard cooler may have broken down.
Open up the case and check it.
Or within the 15-20mins download a temperature display software click here and tell us the results.
If it is this then you will need a new CPU cooler. We can help you here too.