I have a 94 civic that has recently started having some overheating issues. It seems to run fine until I hit a freeway for a bit, then when I exit the freeway the temperature begins to fluctuate rapidly. In a minutes time the needle can go from hot to middle and then back to hot. The coolant is also flowing back into the resevoir tank. Both the radiator cap and the overflow cap have been replaced, and then retested and are fine. There are no leaks that I have been able to detect. The fan does turn on but seems to blow very hot (almost painfully hot )air (not that I know what temperature that air should be). The garage I took it to said the thermostat and sensors (?) are fine. They also said the only other thing that it could be is the head gasket. Could it be that and could it be anything else? Please help!!

It's gotta be either the thermostat or you are leaking somewhere. Are you sure you're not running low on coolant?

Yes, coolant levels are fine.

Anybody?

if the fan runs at normal speed and is not struggling,try crossing out the rad fan with a jumper and see what happens,no overheat-it will be fan sensor-still overheating-head gasket-water pump-blocked water ways.

Thermostat sticking would be my guess. Just does not stick all the time.....