Miner Keeps resetting overclock..

Im using afterburner, and I'm working on lowering my voltage. 2 of my 3 cards, I believe they are both 290x after about an hour they are resetting their voltage and core clocks. normally if they fail they just stop working I thought...at least that what happen in the past. Are they under to much stress and resetting themselves?

Thanks

Edit

Actually the core clock is staying where i clocked it too. just the voltage is resetting.