1987 Oldsmobile Delta 88 Car Hesitation

Engine Cooling problem
1987 Oldsmobile Delta 88 6 cyl Front Wheel Drive Automatic 120K miles

I have an older car but low mileage for as old as the vehicle is, it has had great care, regular maintenance on the engine, oil changes, spark plug changes, etc but am having a problem that no mechanic has been able to diagnose. After the car sits at a light and idles for the period of time the light is, upon acceleration of the car, the car hesitates, occasionally backfires but not often, the car always takes off but very slowly and sometimes acts as if it will stall but usually does not, occasionally it dies but will always immediately start back up. When driving on the highway it runs smoothly and just fine but when sitting in traffic and idling, the car putzes and acts like it doesn't want to accelerate. The fuel pump, throttle position has passed inspection when put through and engine diagnostic test, what suggestions do you have since paying for an engine diagnostic was pointless?