1998 Legacy 2.5 overheating

Been working on 1998 subaru legacy 2.5 w/156,000 miles. Replaced thermostat, seemed like it was sticking. Customer picked up and headed out of town. Overheated again and they took it to the dealer. Same diagnosis-they replaced thermostat. Overheated again. Had radiator flow tested-no problems. Checked for exhaust gases w/chemical block tester-none found. Replaced timing belt on hint from Mitchell tip aboutnot enough tension on water pump pulley. Checked impeller on water pump. Overflow tank is spilling over when overheated. When pulling hills, temp is around 185, when topping hill and on a flat temp starts rising. Downshifting and getting rpm's up temp starts dropping back down. Cooling fans are cycling properly. At a stop temp willrise to 217, if you bring the rpm's up to 2000-2500 temp comes back down. Seems like a flow problem but cannot see anything restricting flow. Could head gasket still be a problem even though no gases are detected?Could tensioner maybe a little weak and the pump not turning properly? When overheating upper hose is hot and lower hose is cold.