It could either be software based or hardware. I've had a couple of boards exhibit this problem and it was in the MPEG decoding section that caused it.

You can try a reimage with the latest release if you have nothing to lose, but if you have too many recordings you want to keep then I suggest trying a test hard disk first. If you have access to a smallish drive (say 4GB and up) this will be good enough to rule out the hardware.