I grow increasingly disappointed with memtest lately. It's OK with actual faulty modules, but it won't pick up memory with incorrect timings or voltages.
If you can get into the BIOS without problems, manually adjust the memory settings to match the specs for your RAM.
If that doesn't resolve the issue, check to make sure all fans are spinning.
Now that I've said all that, my opinion is that the motherboard has failed and needs to be RMA'd. Happens sometimes.