A background running application could cause that (security, disc management, etc). If you have a fire wall, try disabling that (including the windows one).
I guess you do have the newest drivers since you are after reinstall...
Make sure the integrated GPU is fully disabled. You might want to upgrade that though, including your CPU, heh.
If you have some energy saving utilities or something (like AMD's Cool'n'Quiet), try disabling that too.
That's all I can think of so far.