Bleh...... Thermo problems????

[AlexSchuSupra]

Ok so I have an 87 Supra N/A with the 5 spd. Anyway I bought this car with 100,000 miles! When I purchased the vehicle it had to have ALL belts replaced and a new water pump (along with gasket) radiator engine flush w/ chemicals and a new thermostat. Anyway the car is doing great except for this missfire due to the cyl 3 gasket but that is an easy fix. But just recently my thermo gauge had been flipping out on me! This car NEVER got hot no matter how hard I run it! But now from time to time the thermo gauge will go ALL the way up to the top and then spike back down to more normal temperatures. Anyway I checked the oil level just to make sure I was not stupid and gonna blow a head.....full oil! Coolant level is also full and its clean and new! So i am coming to the conclusion that the thermostat they have replaced may have been faulty! The engine still does not seem to get hot and even when the gauge is all the way up I can stand with my face over it and not feel the heat! So I was just trying to get a second opinion on the matter and find out what you guys thought.
Thanks!