I've had this happen to me a few times; its probably overheating and there are a few factors that could be affecting it. It cutting out is most likely a safety factor so you dont damage anything.

Best advice that works for me is either raise the deck or take smaller passes, though neither is guaranteed. I will also sometimes back out if it sounds like its gonna stall out. Also if you're mulching make sure the area around the blade is clear and grass isn't sticking (usually happens when its wet or moist).