High Temp. Gage up and down

My Oldsmobile temperature gage inside the car will show over heating with needle going to max temp and seconds later going back to normal temperature just as quick. I have replaced the intake manifold gasket, temperature sensor, thermastat and radiater cap. I also have tried to burb the system by placing a no spill funnel on the radiater fill inlet and let run for over an hour. The car would burp every few minutes when what I believe was when the thermostat opened and that is when the temp gage inside would go to the max.