Fuel burning issue

Hi there,
I have a 1991 Subaru Legacy L, sedan, 4wd/auto, 2.2L. It has 192,000k a huge chunk highway cross country.

Well heres my issue, for me its a mind boggler! I've owned this car for almost 3 weeks now, and just recently the check engine light went on. Well I checked the usual and it wasnt anything I could find, then went off a bit later. Well a few days after that, I was heading out to lunch, when I was going up a hill, the car randomly ran out of power, all electrical worked fine, just the car couldnt move past 5k. So I took it into canadian tire and they said it was a sever transmission issue, well I grabbed my keys and went home, car started up normal and no issues for the next few days. Well I was taking my fiance again out to lunch, the same issue happend again, loss of power, car didnt want to move past 5k, I know its not the tranny, but I changed the spark plugs and mas sensor, that helped a bit to get it home, but still everytime I press on the gas the rpms drop and I loose power, could it be the fuel filter or injectors?