I have a 91 Firebird Formula. I have the electric cooling fan by-passed to run constantly. The car does not overheat idling, but does while I'm driving. The problem is not coming from the water pump or thermostat. Does the outlet at the back of the intake need to go back to the radiator? Does my coolant flow roll off the top of the intake?
I bet your lower radiator hoses is collapsing while you are driving. You should be able to check this when everything is up to temp, then rev the engine while watching the lower hose. It will be very noticeable if it does, as it will flatten out when the pump gets a good suck on the system. The radiator hose can become weak over time and this is the inevitable outcome if it hasn't been changed recently.
Thanks for the feedback. It was while posting this question, that the answer came to me. There was plug in the back of the intake by my distributer that runs to my heater core to the radiator that allows the motor to cool. It has a plug in it. I removed the plug, put in a nipple, now it's running nice and cool!
There is another possibility. The radiator could be clogged. I have seen this many times. The easiest way to determine this is to run the car down the road until it starts to overheat. When it does, quickly pull off the road, shut off the engine, open the hood and put your hand on the radiator core. The CORE, not the tanks. You'll have to reach through the cooling fan blades. If the car doesn't have AC you could go from the front of the car, but that would be unusual. Check it in several places if possible. If it is cool to the touch, it is probably stopped up. It won't show up at idle when less cooling (less flow) is required, but will be an issue at higher speeds/RPM's. The only way to repair this is to replace the radiator. You can try to flush it, but in my experience if it works at all it won't last very long. Good luck! Let us know what works.