Diablo 3 Random Video Stutter - Troubleshooting

So my brother and I are playing Diablo 3. He's using my old computer, which is about 3 years old and played the Beta just fine. Now, I upgraded to a new rig and the old one is pitching a fit while playing the full retail client. Here is a video of the issue:

http://www.youtube.com/watch?v=22bjFKKRrlM

Sporadic video stutter at an insufferable framerate. It's not occurring during video intensive combat or anything - it happens even when standing still every few minutes, and lasts for (approx) 60 seconds.

Someone in comments suggested overheating. CPU is peaking at 48-50 degrees Celsius when the stutter begins happening, which I would not define as "overheating," but he's giving the CPU & GPUs a good spray clean right now to see if it helps. Should I try a new thermal paste? I don't have a temp gauge for the GPUs. Here are the system specs:

CPU - 2.8GHz AMD Athlon X2 5400+
RAM - 8GB (4 x 2048MB) DDR2 PC2-6400 800MHz
GPU - 2x NVIDIA GeForce 9600 GSO SLI

A friend is also playing Diablo III with a system running a single 8400 graphics card and 2GB RAM. He's not having an issue.

I thought maybe our HSI bandwidth was at capacity, so we upgraded and that didn't solve anything.

I'm open for suggestions, especially some that I can try without buying new equipment.

UPDATE: We removed one of the video cards (no longer SLI) and this solved the issue... for a day. Last night we were playing and it started to lag again. So at this point we're thinking it must be overheating. The question is: the CPU or the video card?

UPDATE 2: We've not experienced any stutter in the past 2 weeks. I think the problem was in fact overheating, so as long as he keeps his background activity to a minimum, it hasn't been a problem.