On my 1986, the temperature will slowly move up to the high end at cruising speed. After a few minutes, it will move back down to normal. Then it may or may not repeat. All in a short five mile drive. Any suggestions ?

 

has the thermostat rear seal been replaced?
Has the thermostat been replaced?
and a new coolant cap and HCV and short hose.
also swap in a new temp sensor and solder on new female connectors.
check the timing belt tension,
check the cooling fan clutch

 

Nothing changed but I let it sit for about 3 months. Everything was good before I let it sit.

 

Get an IR gun and check the temps on the car at idle and under load. Could be a faulty sensor or connection.

 

Thanks. It seems to me that with a spurious and repeating problem, it has to be an intermittent problem. Not a permanent failure. Most of the time, temps are normal. It doesn't care what the engine is doing. I.E. It doesn't occur idling at a stop light. It has come and gone on a straight 3 mile run at 45 mph.