91 240 Auto w/ 125K...highway driving 65-80MPH's my temp gauge will do 2 things..first it will bottom out like the car has been sitting ice cold, then it will fly up and almost into the red. Is this possible?..I have changed the thermostat, but that seems to have made the temp swings worse....Once I blast full heat the gauge will drop down to 1/2 line. I don't want to destoy this car..where do i look next? THANKS!!!