DPCs have caused me great problems as well. I’ve spent half an year, trying to solve my DPC problem. Something kept eating up my CPU, when I played music, played games, almost anything. I was on the verge of pulling my hair out, till I found out about the Microsoft SysInternals Forum for the topic.
See, the problem was that I was having DPC problems when I played music or games. When I opened iTunes and played some music, the DPC usage spiked to 3-6% on my AMD Athnlon64 3200+. Similarly, while playing games, it could reack upto 30-40% (yet to confirm).
So I could make out that it had to be the audio driver (and maybe the graphics driver). So I tried re-installing the stuff from the original CDs I had got with my motherboard. I did that, and now it works fine, at least the audio. If you have any solution for the video problem, please tell me. Post a comment.