I got the same issue with spikes as you guys, and posted a thread earlier as well. Its a really annoying problem and pretty much the ONLY problem I have with Cubase at the moment.
I wondered what gpu you guys have, as in my experience it seems like the issue mainly happens when I move around windows etc, in particular the piano roll. I have an Intel 4400 mobo gpu, which should be a good thing regarding less bloated drivers etc, but a potential problem may be that this kind of gpu in fact is built into the cpu (I think).I really want to avoid putting new fans in my cabinet, and if I DO buy a new graphics card, there really is no guarantee it will work. I dont have these problems in Ableton Live whatsoever, and I think I didnt have them with Cubase 7, so I think Steinberg did something with their graphics engine moving to 8 and 8.5. Generally the performance in Live, with the same plugins, seems to be better.
I think I will try using one of my graphics cards in my gaming pc with 8.5, just to check if this makes a difference. If not I seriously think that Steinberg needs to do something about this issue, and hopefully in the likely maintenance update coming in not too long. Nothing kills creativity like needless problems like this.
Edit: Ah. I see that shadowfax has a Radeon. Not good