loses power periodically, not all the time

Have you checked for diagnostic trouble codes in the engine control module?

I have come across this with all kinds of different cars but with the Subaru's I believe that there may be a problem with the Knock Sensors most commonly. The heat appears to get to them and make them crack. Located on top of the engine, behind the throttle body on the block. This affects the ignition timing and can cause the overheating that you mentioned. Something else that needs to be checked is the cooling fan opperation and thermostat as well. I after warmed up, both fans are not coming on this will cause overheat durning slow speeds, and you might also check the temperature of the upper and lower radiator hose after warmed up completly as they should both be hot to the touch, if one is cold you may have a thermostat sticking. Good luck.