New transmission and a loss of power

Background: The engine was replaced maybe three years ago (has around 14,000 miles on it) and I just recently had a new automatic transmission put in. Originally when the transmission went out around five months ago I attempted to have someone replace it for me at home with a transmission we found off a crashed Subaru. It did not work because it was not the same year. So we tried to fix the original transmission and put it back in. It would make it around the block and then the all wheel drive would trip out and the car would slip gears and then stop. Then three months later I took it to an actual mechanic and they swapped it with a new transmission from Subaru, however the second I drove it off the lot it would slip hard between gears. So I took it back in. They replaced it again and it is now my current transmission.

The problem now is that it slips slightly when speeding up and I have noticed she does not have the power or speed that she used to have. I am not sure if it is a transmission issue again or if there is another part that I need to replace because everything but the engine and transmission are original to the car.