Good morning...I have a 97 BMW 328i. Sporadically it overheats.

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Good morning...I have a 97 BMW 328i. Sporadically it overheats when driving mostly when I have the AC on. If I leave the heater on low the needle stays in the middle. I've checked for leakage - None -, I've checked the top hose to see if the thermostate is operating. It's warm to touch. I've checked the clutch, pump, and belts and all seem okay. Just recently, I ran the car in park w/o the air on for 15 minutes and then 10 minutes with the AC on and then turned it off for another 10 minutes. Needle stayed in the center. Also, I've bleeded the radiator correctly. It's crazy and over heats only when driven. One other thing, I've taken the cap off the radiator, started the car to see if the radator collant is ciculating. It doesn't seem to be doing anything. Calm as could be. Could this be my problem? Please help. What is causing the car to overheat only when driven?