1992 Pontiac Firebird base with 3.1L

First start she fires off instantly with RPMs jumping up to about 2k then dropping to 750-1k instantly. after warming up she will drop to about 600rpm in drive and around 700-800rpm in park. when warm she is a little harder to start. she will catch and the rpms will sluggishly increase to standard range in comparison to a fresh start.

prior to warming she behaves great but once warm she will make a rumbling noise from the exhaust with an occasional backfire and slight fluctuation in rpm. This behavior happens consistently at around 1.8-2k rpm range at speed specially when im doing around 75mph. in first gear if i slowly accelerate she stumbles a bit before shifting into second. I am in the south and its summer so the temperature is complete hell but the car runs at about 180F but i will build a decent amount of pressure in the fuel tank.

replaced/checked parts:
distributor (timed as per specs)
icm
pickup coil
ignition coil
egr
ecm
dist cap
rotor
spark wires
spark plugs
tps
iac
MAP sensor
IAC (cleaned and reinstalled)
PCV (passed check)
fuel pressure (checked and held at spec)
fuel filter (replaced ~yr ago)
checked for vac leaks but couldnt find any