Why do I steadily lose power in 5th?

[Captain Ford]

I have a 99 Subaru Impreza Outback Sport Wagon with a 2.2 engine in it. It's got single header exhaust y pipe. When I'm going up a steady highway incline in 5th I begin to lose power. Even flooring it doesn't help very much. It doesn't happen in other gears. Even in 3rd on pretty steep hills but only in 5th on gradual hills. I'll be going like 75-80 and it will drop to about 65. Can someone help me out with this? 

Also is an EJ22 from a 2000 easily transferable over to the 99? Is it a straight swap? This current one in the 99 has been nothing but problematic for me. The guy I bought it from said its also not the original engine. But I have a good EJ22 from a 2000 that I wrecked that's at 107,000 miles and it was a fantastic engine.