my car is a 1982 kjet car without a compensation board for the thermostat.

The thermostat is normally in the 35% percent mark...but heres the catch

on hot days( 80 degrees) it goes up to 50% on the highway under higher speeds. Then if i am still going fast..and i coast..the temp drops to like 20% then after i engage gears it climbs slowly back to 50% on the highway....

One cold days(60 degre weather) it stays put at 35% even at highway speed driving

In street level drivign it is always 35%.

Radiator is new 2 years old and no obstructions
Fan clutch seems to be good
New thermostat, new water pump
belts are fine
System i flushed recently..new coolant
No air bubbles....50/50 coolant/water mix


THe old thermostat woudl flux wildly as well...except that would go near to red from 50%....so i'm stumped


My two suspects are a bad thermostat?? and my second suspect is perhaps the fan clutch is not disenaging in cold days...and somehow loosening up when it gets hotter??? but 20 degrees shoudl not be doing this


I'm afraid these hug fluxes will damage my engine if i leave them be...
Any help:)

thanks