1988 GMC C1500 don't know where to look next

I have been chasing this issue for a while and can't pin it down. Engine starts and idles well. Between 25 and 40 mph engine spits and sputters until it shifts into 3rd. Runs great for about 15 or 20 miles at 55 mph or higher. At this point it starts bucking, spitting and sputtering. Sometimes I can push the pedal to the floor with no response at this point and sometimes the rpms will skyrocket while the tranny drops to 3rd and won't go back into 4th. Once engine cools completely the whole cycle will start over. Strange as it sounds I have found that when the outside temperature is below 70 degrees I can drive all day under any conditions with no problems. As soon as it goes over 70 deg outside the trouble begins. I have replaced egr valve and solenoid, tps, water pump, ect sensor, thermostat, fuel filter, tranny filter and modulator, plugs, wires, dist cap, rotor. Not sure where to go next