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I have a 1997 pontiac grand am with a 3.1ltr. engine. I had a blown head gasket that was replaced. also a new radiator and new thermostat.System was pressure tested with no leaks found. When parked and idling water temps are ok. But when driving in traffic the engine starts overheating after 10 to 15 miles of travel. there is circulation so water pump should be ok.there is no steam in exhaust or water in oil. What else is there to check??  

Radiator cooling fan not working correctly is the first place to look, got a scan tool?  

cooling fan tested ok also.

Does it boil over? or does the gauge read high if it's just the gauge reading high I would hook up scan tool to read actual engine temp via ECT sensor  

It boils over when driving, but not when parked and idling.  system was pressure tested at 14lbs., radiator cap is also new.  If there was a crack in a cylinder wall, would this cause this?















 

Sure best thing to do is have it tested for combustion gas in the coolant, that will tell you what's happening.  

It still could be the water pump. If the water pump is old it could flow water at idle but cavitate when you accelerate. I have seen this happan before. If it was a cracked cylinder you would normally get water in the oil. What temp thermostat did you put in? If you put a 160 in then switch it to a 195 and that should correct the overheating problem.  

I replaced the water pump about a year ago.

Trust me on this, Purchase a combustion gas detection kit from napa, carolinas. or any good auto supply store, radiator shop. You can do this at home. Don't spend another penny until you do this step! I assume the radiator has good airflow...no bra or missing air deflector??