I had almost this exact problem with my car ('96 Lumina). The problem with mine was the gaskets and hoses were nearly rotted out from very little use over the course of a few years before I had it.
When I'd look for parking, temp would almost read max on the gauge. Turned on the AC and it cooled it down a little, rev'ed the engine, cooled it down a lot. Freeway driving was ok. I had the radiator rotted out and a new thermostat put it, but it didn't help.
Sound about right?
Coolant was leaking through the tiny cracks and reving the engine forced an adequate amount of coolant past the cracks. Send it to a mechanic NOW. My problem could have ended up as a cracked block.