I had a similar problem with one of my older rigs. I cleaned everything, put on a V8 cooler (on an Athlon 64), even turned a desk fan onto it on full blast. Nothing helped. So as HK said, its probably motherboard or CPU. I ended up replacing my CPU and board, but if OP has some spare parts he can pop in, he might be able to isolate the problem.
I would recommend taking a look at the 6 core AMD chips (or Intel if one is rich), if his video editing software takes advantage of the multi core setup, he may end up getting better performance.
Another suggestion would be to install some kind of CPU throttling software, and see if the problem persists if he caps it at some percentage.
But I think if you eliminate all the common problems, the most likely one is that the hardware has just reached the end of its life cycle.
I would recommend taking a look at the 6 core AMD chips (or Intel if one is rich), if his video editing software takes advantage of the multi core setup, he may end up getting better performance.
Another suggestion would be to install some kind of CPU throttling software, and see if the problem persists if he caps it at some percentage.
But I think if you eliminate all the common problems, the most likely one is that the hardware has just reached the end of its life cycle.