I am experiencing the same issue (W7 64) on two different computers (one with NVIDIA Quadro 4000 and Phenom II and another with NVIDIA Quadro K5000 and i7 4690). I saw it during the last year with cubase 5.5 and now with cubase 8). Always the same: Spikes/freezes type of issues in no matter what type of projects.
Interestingly: If I revert to NVIDIA Quadro Driver 312.69 WHQL or earlier the issues dissapear immediately (even without rebooting). Installing a later driver brings the issue back for me.
As far as I know Cubase is using OpenGL drawcalls (i think for the GUI and the video renderer that seems to be running even if no video content is used). I remeber reading a release note or something like that that NVIDIA changed something with OpenGL commands or command handling somewhere around the driver revisions 3xx or so. I also saw graphic driver related entries in the system event log on the older machine that correlated to the spike issues in my DAW. Not on my new one though that I am currently using - but nevertheless the symptoms look exactly the same.
Maybe someone can solve it the same way - though it’s kind of annoying to have to stick to outdated drivers…