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.

[moosens]

My first thought is yes you’ve got from I think 1998-2001 plus or minus 

Not sure about doing a full swap but going to the two cam vs using the quad cam was an option many folks took up back when. 

[brus brother]

So is there no deteriorating catalytic converter potentially contributing to clogged exhaust?

Saw it on a Ford van years ago. Ran it loud and proud to diagnose a clogged cat.

[el_freddo]

^ That was my thought too.

As this issue gets progressively worse you’ll start losing power in other gears too. 

[nvu]

I vaguely recall sohc single port EJ22 is an oddball thing somewhere around 97 when they went from phase 1 to phase 2.  Are the valve covers on the heads the old school ones that have "16 VALVES"  imprinted on them?

\ 

If it's an 99 ecu, it's probably not really meant for those heads, probably why performance is poor.  Good and bad is ecu is fairly simple and doesn't care which block is in there.  Swap in the 2000 block, it's what the 99' ecu wants over all.

Or if your state emissions doesn't care, drop in an 97-98 ecu and it might fix everything; if the current block has good compression and no mechanical issues.