I'm not really sure if there is anything I would say to try that you haven't yet
One thing I noticed though - did you install any windows updates before you installed your drivers? I don't know why but that could give some funky results. And are you running the latest BIOS version?
Also - you could try running the benchmarks on just the bare Windows install (with drivers, of course), without any of MS' updates.
Hope you get this sorted!
EDIT : Are you sure that nothing is overheating / clogged with dust / whatever ?