I had similar with a dodgy onboard audio chip. Have you tried disabling absolutely everything in bios? Audio, usb, firewire, lan - everything.

2k8 doesnt have the windows audio service, desktop composition / themes and directx as standard. Thats the only difference between it and 7 in this situation.

If disabling peripherals in bios doesnt solve it then try disabling the windows audio service in 7 and see if that does the trick. If not then disable themes and desktop composition too.

Just shooting you a few thoughts - definitely a tricky one to find the cause of. GL dude.