Definitely do the memtest above, though if it fails that it could still be due to the CPU overheating and not the memory at all. I'd advice you to go into your BIOS and check the CPU core temperature. If it's at about 60C or above it could be the cause of your problems.

This sort of issue is not always hardware however, and may be due to a buggy device driver. Post up your system specs for a start. You may need to post minidump files in order for me to analyze it. This will pinpout the process that causes the crash.