Subaru 98 Outback still overheating

The car has 152k on the engine. The initial overheating resulted in coolant spraying from the cap or overflow. The radiator, water pump, t-stat, various hoses and cap (timing belt as well) have been replaced. The fans both come on. I’ve been reading about the head gasket issues but I’m not seeing the common signs of a leak (foam or oil in coolant, milky like crankcase oil). I plan to have the coolant tested for Hydro Carbons first of the week. The heat gauge will remain fixed at about half way until the car is driven at highway speed for 5 to 10 minutes. When the gauge starts to move it transitions rapidly and will reach the “H” if I don’t reduce speed and stop. The gauge will then return to normal as quickly as it went up. If it reaches “H” this has resulted in coolant releasing from the cap/overflow area, but escaping coolant hasn’t happened since I put a new cap on this morning. On the test drive this morning I drove at highway speed (75mph) and the gauge didn’t move until I came to a stop on the exit ramp. It rose quickly to about 3/4 at which point I revved the engine from idle to 2k and the gauge needle changed direction returning to just above normal. It was not apparent to me that any coolant leaked out. I’d like to get 200k but my confidence is shaky and if the head gasket is going or gone then it may be time to look for a replacement vehicle. I would consider keeping this car for short distance around town driving as everything else is in relatively good shape. Thanks for any help.