would removing the air system and cat change the way the car runs

its a 1988 camaro iroz 350 tpi.

I have rough idle, hesitation, flucuation in the idle up and down from 600 to 1500 rpm sometimes not always

tps and iac are fine, o2 sensor may be old.
fuel pressure at 30psi key on and 41-45 psi at idle
new egr system
brand new rebuilt motor

 

I had the exact same problems, turned out to be 2 things. 1st my bellows had a small crack in the bottom that was leaking in air after the MAF so the car was stumbling real bad, because you say you sometimes have this problem I would check your intake for leaks, the O2 sensor reads the lean condition and adds fuel for the air leak, which would explain it coming and going, but then I would get an occasional backfire, but I have no cats so its easy for it to backfire. My other problem was that I lived in a dusty area and my MAF was a little screwed up, I tried a new burn off relay, and it did nothing, so I wanted to clean the wire on the MAF, well after about 25 people telling me exact conflicting info on how to clean it and that I would ruin it if I did it the other way(all directly conflicting each other) I just bought a new one. All my problems are gone now.

 

i also have poor mileage

 

Have you tried a simple tune up?