Need Help on Overheating Please

Hi all,

Here it goes. The car is a 79 coupe with All motor and suspension from an
86 GT. Converted that motor to carb. Stock Water pump , Stock radiator,
Mr. Gasket 180 thermostat. I installed an entire new gasket set in the motor
before dropping it in. It has been running perfect until a few days ago.
Temp gets way hot, just past the L in NORMAL. I completely burped the
system , there is plenty of flow from the water pump. (it is installed
correctly and spinning in the proper direction) The thermostat is definately
opening. (yes that is installed correctly too) The fan clutch is the only thing
that fails the test. It spins too freely by hand when engine is cold or hot.
This is the wierd part. It does NOT overheat at idle, only when driving.
I thought it was supposed to be the other way around.
Also there are no coolant leaks (coolant in oil, or vice versa, etc...)
I will definately replace the fan clutch , but if anyone has experienced
this too, please let me know if I should look elsewhere also.
Comments are welcome too.

Thanks.