Ok so car has had acceleration issues and throwing P0300 codes, after some prior posts I read I pulled the front O2 sensor(before Cat) and it had lower power but it was running way better and no more P0300.

Then I put it back in, dumped some cat cleaner in the gas tank, and ran fine for a couple weeks.

Started up again so I pulled the O2 sensor again. Rand for about 5 mins like before, then a major power drop and barely limped it home.

Is this just the Cat getting even worse, or is there some other issue potentially that I'm missing.

Also not sure if it's related, but my heater core was leaking, so I bypassed it for now, already ordered a new one.

It's a 2006 LS 2.2L

 

Perhaps you have a clogged cat... Had one years ago in my Olds cutlass. I replaced it and it made a world of a difference. However, my olds made an audible rattle from the cat so thats what gave it away.