To be honest I don't know why this is happening, sometimes I'm getting the very same behavior on my Win7 x64. Not sure if it happens on other OSes (32-bit ones) but all reports so far have been from 64-bit Vista/Win7. And I don't remember seeing anything like this on a 32-bit XP.
I tried to figure it out but there just doesn't seem to be any pattern in these GFlops changes.
Bookmarks