I have a 99 dakota that was just in the shop to replace a sensor (sorry, can't remember which right now) that corrected a problem with going into high RPM's before shifting. That problem is fixed, but the day it left the shop, it started overheating. It only overheats right after I get gas!! The shop said it has nothing to do with what they did (of course). The temperature gague goes into the red, and the heater starts pumping out cold air. After 5-10 minutes of gentle driving/stopping to let it cool down, it goes right back to normal. Any ideas??

I think you have air in the system have you checked the level as it should have dropped when it reached operating temperature.