haven't posted here in a while. but this topic caught my attention.
I take it that the first card the op used was an nVidia card? nVidia are having some serious problems regarding DPC spikes with their drivers.
the spikes on desktop machines seem to happen when you start doing something a bit more GPU intensive like watching a HD video on you tube. that's how I test it anyway. that's when the high spikes occur.
on laptops it's a whole other world of pain. but using this tool (http://somemorebytes.com/wp/index.php/s ... pmmanager/
) you can get it somewhat under control. naturally, YMMV.
the problem is the powermizer feature nVidia uses. there's a very long thread on nVidia's forum full of people having DPC issues on their laptops.
I have no idea why nVidia a) apparently are unable to fix this problem and b) why they apparently don't care about it. I find it rather baffling.
a good way to double check that it really is the graphics driver that's causing trouble is using this tool: http://resplendence.com/latencymon
some of you probably already know it. very helpful.