Edit: Now it looks like another GPU is doing the same thing, so narrowing down to either whole card going, or power supply. In cgminer I have gpu 0-3. And 0 and 2 are now problematic, are these the same card? And the clocks on this new problematic core are jumping all over the place and the core as listed in the OP is stable at 725/1000. So the card could have a power issue or something with VRM's?
Interesting thought about the power supply. One of the reasons I only buy SS psus. When debugging mining issues i never have to question my power.
edit: since you have 2 cards, and you suspect psu. take the other card out, use the known good pci wires from that card and check to see if the 'bad' card still acts up.