I've had the same issue with my 92. Every time I turned on the heater motor the temp gauge went up. I changed the water pump and went through 4 or 5 thermostats and flushed the system several times and nothing changed. I checked the water temp with a hand held gauge and found that the water temp was well within specs. It was just the gauge giving me a false reading. The top hose was hot and the lower one was just warm. Try a new gauge if all else fails.
rem: