Mining stresses a rig in many ways, so just because it doesn't freeze up when not mining doesn't mean there is a problem.

I would start by shutting it down completely, re-seating all video cards, the RAM and reconnecting all cables to eliminate a loose connection somewhere. I would also check to make sure there is not dust or dirt build up and that the cpu heat-sink is secure to rule-out common heat trouble spots. Turn it back on and monitor it for awhile to make sure a GPU is not overheating causing the rig to lock up.

If it passes all this and you are still having issues, the next thing would be to try and remove and reinstall the video graphics drivers.

After doing all of the above, the next things to try would be removing one card and seeing if the rig runs well, repeating until you either found the problem card or eliminated them as the cause. You may need to let the rig run for a day or two in-between these steps, I am not sure how often your rig was freezing up before, so wait at least that long, and maybe double it to be sure.