I doubt its the head gasket. It never gets too hot. Most of the time it
runs fine and doesnt get hot. Why would turning on the heater make the
temperature go down immediately if it were headgasket.

Thermostat sounds interesting. Does the heater somehow bypass the the
thermostat ? How difficult is it to replace the thermostat ? How much
should it cost to have someone do it (ballpark) ?

Thanks

G