[SoTxButler]

I had this problem with a car that had a 351 in it. As soon as the car started to move, the temp would start down and then up again as it came to an idle.

Every time I opened the hood, the fan was running...so I never thought to check to see when the fan came on.

After changing the thermostat a second time, I installed a mechanical gauge and was watching the car idle and because I was watching the mechanical gauge, I noticed that the fan didn't turn on until after the temp went past 200 degrees.

I checked the thermistor in the radiator and found it to be defective, it was working and running the fan, but too late. So, air flow cooled while rolling and temperature rose while idling and the fan was turning. I installed a fan controller with a probe for the radiator and set it to turn on the fan at 180 and voila...fan starting turning at the right temperature and no more spiking temperatures at idle or slow speeds.

Check to see what temperature the fan starts running.