Engine Overheat Problem

Have a 68 Buick with a 430 V-8. The "Hot" light comes on when the car
has been driven and then sits with then engine off for approximately 20
min. If the car is started and the light comes on, I can rev the
engine up a bit and the light will go out. If I drive it below 75mph
the temp light stays out...if I drive it fast...then slow down and
drive 65mph...the light eventually comes on.
The engine was rebuilt about 2 yrs ago, the engine has a new radiator,
water pump, themostat and temp sensor. I took a chance and replaced
the water pump....no such luck. The sensor was checked to make sure it
works...works fine. I am thinking about replacing the radiator and
finding a differant fan. Any thoughts? Please contact me at
.

Thanks!