Windows has a memory diagnostic that you can switch on while its booting, find it and run it. If it fails, then you have a memory issue. Also another way, that is quicker, test each RAM stick individually on the motherboard and run Prime95 for each. If there is a problem for each, try another slot. If it is the slot, you need to rma your motherboard. hopefully its just an individual stick!
Resetting the BIOS to default fixed it. Now I can run Prime95 indefinately. Thanks.