I have been having this problem just recently.
Noticed it on Cubase 5, after semi heavy sessions, editing in the key editor or drum editor, when I drag around midi events, the screen goes blank white until I unclick the mouse. It also happens when pressing command without clicking the mouse. It’s a blank screen but I can still see faint gridlines. If I continue to work, it spreads out to the arrangement page, where clicking on a part makes the arrangement window go blank.
After upgrading to 6.5 I have just noticed it again. It doesn’t happen straightaway, it’s generally after working in the midi editors for a bit, then it goes a bit glitchy until full off blank screens everytime I try to adjust an event.
Never had video problems before, the only changes to my system are installing 6.5, along with normal disk utility maintenance, upgrading ELicense. I also had to replace 2GB memory 2 months ago. I’m wondering if I’m beginning to get hints that my 2008 Mac Pro is breaking down, as there have been other smaller “issues” that are cropping up.