I need to start off by saying that I have read a vast amount of information on this topic in the archives, but none of the fixes seem to apply to my problem.
Let me describe the problem I am having in my 89 244:

I can start the car and drive it after it has been sitting for a while. It takes the temp gauge the normal amount of time to start moving. However, the needle never reaches halfway, just a little below it. Then, after driving for about 15 min w/ the air condition on, the temp gauge will rise about a needles width from the red. I then cut the heat on full blast, and the temp gauge will drop back down its normal position, and I can drive the car home. What is weird is I can let the car idle in the driveway after it overheats w/o the heat on and it will not overheat. It only seems to overheat when I am driving.
I bought this car about a week and a half ago. The previous owner had replaced the thermostat, flushed the coolant, got a new timing belt and new external belts(alternator, ac compressor, etc) and replaced the hoses. He also has taken out the radiator and cleaned it a little while ago.
I would think this might be the water pump since it is about the only thing that hasn't been replaced, but if it was failing, wouldn't it have trouble creating heat to go through the vents?

This problem has baffled me. Maybe time for a new radiator?

What do you all think?

Thanks alot.

Whitt Wilkins
89 244dl