Sorry Stephen, you did mention Linux above but I missed it during a skip read. Bad habbit.

It's good news Linux is running ok - might be a driver issue or hardware conflict in Windows then. Or perhaps Linux isn't stressing your GPU and therefore PSU as hard and the issue isn't showing itself.

If memtest ran fine, then that means your ram is fine. It spouts out errors fairly quickly with dodgy ram.

It does sound more of a Windows issue or hardware/driver conflict. Also check out your hard drives using HDtune for any problems. Sometimes a dodgy SATA cable can cause strange issues, and can go bad just out of the blue.