SOLVED!

So I got the cat checked and it was fine. I didn't put paint thinner in the gas tank.

It ended up being the timing. The guys that originally fixed it said it could not be the timing because it idled fine. Another mechanic I took it to said that is not true and adjusted the timing. It worked. Shoulda went back to the basics!

Hey guys, I'm a big computer nerd and not much of a car person but I would really appreciate any help from this community on my car issue.
There is a Too Long Didn't Read at the bottom.

Little background:
I got a 2001 Nissan Altima Limited Edition GXE from my dad's friend last year in June. Car ran great. Only thing he said needed done was a O2 sensor put in (the check engine light is still on) but I said screw it because the car was running excellent anyway.

The story:
Everything was going good until Fall last year. I was basically coasting home going maybe 20 MPH, touched the gas pedal and my car basically lost all power and acceleration. I could barely move. I shut the car off and couldn't get it started back again. I finally got it started after about 10 tries or so. Got it home, didn't start at all. Got it towed to a shop.

Mechanic said there was very little compression. They did some digging and found that the timing chain snapped. They replaced that, spark plugs, fuel filter and just basically a whole tuneup. Paid about a grand.

Got it back, idled fine, and ran great with no loss in performance or acceleration until a couple minutes later on the way home. It was REALLY sluggish. The best way I can explain it is that it loses insane sensitivity to the gas pedal after being run for a minute or two. Oddly enough, it seems to happen when I make a turn and it just loses insane power. I have to keep pressing down on the gas, way to far just to hit 35, then I can just coast. Hills are a huge problem!!!

On the highway ramp this is a big issue, sometimes pressing down all the way and it finally 'catches' and revs up to 5 RPMS but even then it doesn't have that acceleration like it had before.

I took it back to the guy and that's when he put in the fuel filter. Still same issue. Only thing he said could be wrong is the fuel injectors are dirty so I bought about 4 injection cleaners and even a different brand and spaced the treatment out when I filled up the tank. Still the same issue.

TL;DR: Car very sluggish after replaced timing chain with almost no acceleration after run for a minute or two

Anyway, If you read all of that I salute you and really appreciate your help! Anymore info you need don't hesitate to ask.

Thanks again

If you have an automatic, check your transmission fluid levels. I overlooked this, and it actually was the cause of my issues - which were very similar to yours. Make sure to check the level when the car is running, because it will seem falsely high when the car is off.

Thanks for the advice man.

I finally went to autozone to get the service engine code checked and here are the ones that it put out:
P0420
P0455

I'm going to put a gallon of paint thinner into my gas tank when it is at half a tank and run it for about 120 miles. If the catalytic converter is clogged, hopefully the paint thinner will clean it out.

I've never head of the paint thinner technique before. How safe is that? I know lots of folks that have had luck with Seafoam.

SOLVED!

So I got the cat checked and it was fine. I didn't put paint thinner in the gas tank. To answer your question, it was recommended by a very experienced car mechanic on youtube

It ended up being the timing. The guys that originally fixed it said it could not be the timing because it idled fine. Another mechanic I took it to said that is not true and adjusted the timing. It worked. Shoulda went back to the basics!