Random thoughts without an answer:

What year are we talking about? Every time I've had an issue with an O2 sensor on a Subaru it has thrown a CE light. Is it temperature related? Is it like a cylinder miss, or just a lack of power? My old 60s carb'd pickup did something exactly like this, in my case the PO had the thing set so lean it would fuel starve but only when it was hot and had been sitting for awhile. Cold it was always fine.

 

it a 99 foz s. i cant tell if it throws a CEL cause the light is either burned out or it was pulled cause of the knock it had (fixed headgasket). temp wise, it happens at all temps. mainly when its cold but also its mainly during the 1200-3500rpm range. as soon as i get in the higher revs 4k plus, it runs great. the car also seems to "lack power" when it shifts, goin from the good running high rpm, to the half power low rpms

 

so this is really startin to puzzle me and my mechanic. today while on the highway for a few minutes (after a 30min drive and sitting for an hour) i came to a slight hill at 70mph and halfway up the hill it started to feel like it was running lean. hit the gas pedal and it made more noise, just didnt accel. it lasted like this for a few miles then it started to run like a champ afterwards. im starting to lean towards the fpr or even fuel pump. but im still lost on which direction to go first

 

How many miles are on the car and how old are the O2 sensors. I don't trust O2 sensors past 100K miles. If yours are still the originals, you probably have way too many miles on them and you should replace both of them. The front one is a real pain to get to.

 

car has 162K miles and i bought it with 145k miles. im gunna leave the o2s alone for now. i need to replace the whole exhaust and will be makin most of it myself. most likely a single high flow cat, 2.5in, into borla or ansa muffler. but thats gunna be a few months down the road

 

Could it be the fuel pump?

I had a similar issue in the XT when it would struggle first thing in the morning and would be great otherwise except on some hills.

Turned out to be not enough fuel pressure.

 

Doesn't the CEL light up (along with all the other lights) when you first start the car?
If it's burned out, that's the first thing I'd check. That and check for codes.

 

only code i get is for the knock sensor. i did get one for the evap system, but i unhooked a vacuum hose with it running.