if you bring cards down off of the the OC setting or "boost" in tuner to a default setting and run in SLI is it the same result for you after your 15 min?what kind of frame rates are you getting in game?
possible the PSU crapping out /dropping voltage when it starts to warm up or get load on it if there is a pattern that you go from a cold start or idle and 15 min later the shuttering starts, you have a spare PSU you can throw in to test? just a tho...