Let me preface my post:
I know very little about DVR systems. I haven't build one and don't own one (yet).
but...
I have seen this same issue with computer video cards that are overheating. You can troubleshoot this by opening up the case up and havening a fan blow on the card and see if the issue clears up. A card that is very hot to the touch is also an indication of overheating. A improperly wired cooling fan, a poorly seated heat sink, or a very hot ambient temperature could create this type of problem.