This is what comes to mind for things to check:
- Check your cables. Make sure none of them are loose, damaged, or obstructing fans
- Make sure your RAM is seated properly, your GPU isn't sagging, the heatsink is properly attached to your CPU, etc.
- Watch the thermals and physically check to make sure that your fans are actually spinning when they should be
- Disconnect the GPU and see if it still fails
- Run a memtest
- Try a different kernel (newer is usually but not always better...)
Good luck. Hope you can get the gremlins worked out soon!