I have a 1991 chevy G-20 van that I recently bought. When I drive it for approximately 120 miles with the air conditioner on, when I come to a stop sign or red light, the motor tends to cut off. I think this van was setting up for about 2 years prior to buying it. I have changed the fuel filter, the oil and oil filter, changed the transmission filter and fluid, and tuned the engine (plugs, D cap, rotor, wire and coil) yet, the problem persists. Do anyone know what is causing this problem and how to fix it. It also appears that the motor is overheating.