have you tried a cooler thermostat? PepBoys carries an 82' thermostat that I've been running for over a year now. Works like a charm.......I'll get you the part# this evening if you want it. I bet yours is just sticking or slow to react.
Also if your worried about losing gas mileage or bad emissions with the 82' don't. My gas mileage is still great and last November the car blew as clean as it did a year earlier with a 87' thermo in it!!