For mine it was the thermostat getting stuck in a partially open state. By increasing revs, I could increase the speed of the water pump to cool it off by forcing more coolant past the partially stuck thermostat. I'll be draining the coolant system and changing my thermostat in the next couple weeks.
I had the same issue where revving up the engine would get the coolant circulating again. But it was because the old thermostat wasn't opening until it got significantly hotter than the 180 degree rating.