I have a 1999 Plymouth Grand Voyager Espresso mini-van that has started to get hot when the temperature outside is above 90 degrees and the air conditioning is running with the engine on idle. If the car is moving at normal city driving speeds it is fine. If you turn the A/C off the engine cools to normal temperature. I have checked the fans and they seem to be functioning correctly. I had the cooling system flushed about 2 months ago and it seemed to resolve the problem but it has returned. The van had 129,0000 miles. Is the radiator in need of replacing? Could it be the thermostat? Do water pumps slow down over time?
Thank you in advance for the assistance.

The answer could be yes to all of your questions. Radiator passages may be blocked by phosphate scale and it can cause thermostats to stick. Water pump impellars can even disintegrate. Reading the link below should help you formulate a plan of attack:

https://www.askmehelpdesk.com/cars-t...tml#post235973

If you have not been using distilled water with your antifreeze (or premix), consider replacing the radiator. It may be compromised but not real bad, yet. Also, check the shroud around the cooling fan. Gaps can greatly reduce the effectiveness of the fan. It's easy and inexpensive to drain the coolant a little and replace the thermostat. An infra-red thermometer may help identify the problem.