I can't MB with this (main) PC, either... after anything from 1 - 30 mins it BSODs.
I've installed the MS debug tools and analysed the crash dumps and it's always a random DLL that 'appears' to be responsible... sometimes it's the nVidia driver, sometimes the SoundBlaster driver, sometimes a bit of Windows itself.

In the end, I gave up and built a second PC from old parts and now run 1+4.