Over Heating 2004 Subaru Outback Base model

2004 subaru outback I have an interesting situation: About a month ago my subaru started overheating on the highway. I pulled off the exit and opened the hood. The reserve tank was full so I opened the radiator cap. This seemed to "Burp" the air and the coolant sucked back in after I started it back up. I did not have any issues again for three weeks. Now after the engine heats up (ten or so miles) the thermostat will start increase, but only at lower RPM's. If I can maintain speed or higher RPM's (neutral while stopped at lights) it will keep blowing hot air and even temperature. Here is what I have replaced to date: - Head gasket replaced at 80K currently at 140k -replaced radiator/radiator cap/Thermostat Yes its the correct large Bore -timing belt, tensioner, water pump -Heater Core The following tests have been completed: - Head Gasket pressure test (passed several times) - CO2 hydrocarbon test on the radiator (Passed several times) - pressure tests on the coolant system (Passed several times) The mechanic is now telling me they have no idea what to do and recommend replacing the motor. Is there something I am missing? There are no leaks, the oil looks good, the coolant looks good....ANY help would be wonderful. At this juncture if it is not a simple fix I need to to run long enough to trade in. HELP!