1997 Cadillac Seville overheating

When we first got the car (sls) it ran fine but then it started to overheat. So we thought it was the headgaket so we bought this stuff called thermaseal and we flushed out the system and did what it said on the can. Did not over heat when we let it itle. Heres the thing it will not over heat when u sit in park and have it at 2, ooorpms for 5-10 min the temp gage stay halfway but when you drive it for a min or so down the street it would overheat. So we replaced the rad and looked at the water pump but the water pump was fine. Filed up the new rad and tryed again soon as we would go down the street it would heat up and fast. I have no idea what it could be. Anyone have any ideas? Thanks