voltage regulator or thermostat??

driving my 86 vette today...the temp gauge was 197F the read 17.4 V....i turned the switch on and off...read 240F with the red light on. for a few minutes, the ac/heater gauge went dead. when it came back on... i turned on the heater to 90F and everything went back to normal. this happened on the way and on my way back home. any ideas???