1.8, 4cyl 5spd AWD with 112,000 miles.  Have replaced timing belts, tuneup, oil pump seals but still doesn't have much power up even a gradual hill.  Have been told to replace throttle positioning sensor which is a pretty spendy part.  Looking for some expert advice from other Subaru owners?

Check for vac leaks, make sure the exhaust isn't clogged at all, give us a little more info on how it runs all other times. Does it miss? Run smooth? Does it have good pick-up when you floor it on level ground?

Can you run a compression test on each cylinder?

Also it's probably not AWD. Those are kinda rare. If you have a button on the stick shift or a lever you move to engage 4wd (2nd stick) it's 4WD not AWD and you need to make sure you don't use that 4WD on dry surfaces such as pavement.

Don't know how the A replaced the 4, but yes it is a 4WD and no I don't use it much in Oregon.  Runs smooth and accelerating on flat is no problem, although I don't travel over 65 on the flat.  It does miss when climbing and I am giving it gas and down shifting to struggle up any hill. Where do you check for clog in exhaust?  I can have a compression test done and someone check for vacuum leaks but that will entail a garage I guess.

Sputtering on a long hill,but running well otherwise is usually a sign of fuel starvation.

Try changing the fuel filter first.