My guess is that the heat caused the water to move away from the block and the pump, sort of boiled in there which caused the impeller to start dry.

Temps are fine now, for an H50. Load at 4.0GHz 1.425v on my PHII 965 is 52c prime blend. The H50 is fairly limited in terms of it's heat capacity.

If you're going to do this, I recommend leaving the pump running, fan off for the first 30 seconds, then turn the pump off until it hits 80c, then pump back on. The point being, don't let the pump sit there at 80c for too long. Then leave it going for another minute with just the pump on. Should be enough for proper flow.