Could be any one (or more) of a few things;
First thing I would check is for overheating. Start by checking the thermal paste is present on the CPU, and making sure that the CPU cooler is fitted properly.
Second option is that the motherboard isn't earthed properly. Check that the mounting points are all present and that the screws are nipped up.
Third, you should be able to check the RAM through Memtest (click here option is that one or more of the components has a fault - a cracked motherboard or faulty PSU are tricky to diagnose without 'spares' to fit and test.