I bought a new PC and installed Windows 7 x64 Professional.
Ran fine for a while, turned it off at night and turned it on in the morning - crashed constantly and gave memory_management bsods (WhoCrashed reported ntoskrnl.exe as the culprit). After being unable to keep Windows stable, I made a Memtest86+ bootable key, ran that and ran Windows Memory Diagnostics. Both gave me errors.
Did a complete reinstall, had the computer running for the whole day, ran memtest and WME and there were no errors whatsoever. Turned computer off overnight.
Woke up this morning, turned it on - again with the memory_management bsods. I could boot to Windows, have it running for like 3-4 minutes and then it would crash. Ran memtest again - hundreds of thousands of errors. After about 10 crashes, I'm now running it without problem, ran WME - no errors (didn't get to running memtest again).
I have no idea what could be causing the problems, and I'm guessing they happen when the system completely cools down.
I can't open the case or I'll void my warranty, so any suggestions before actually taking it back to the store would be great.
My motherboard is Asrock P55 Pro and I have a set of G.Skill 2GB ram.