2005 Town & Country Overheating

My 2005 T&C started overheating. I found a leak in the radiator so we
replaced that as well as the thermostat. vehicle was still overheating so we
purged the coolant system, it runs fine when completely back filled with
coolant but as soon as the cap is replaced it overheats. all cylinder
pressure tests are good. is there a possible leak in the head gasket? My
mechanic was also concerned about subtle knocking in the top end. Am I
looking at spending more than this vehicle is worth if I continue with this
goose chase? are there any suggestions as to what the problem might be
and how labor intensive the fix is?  Thank You for the feedback!!