I've searched just about everywhere and I can't find anybody who's had this specific problem so I'm getting desperate. Car in context is a 1994 Camaro 3.4 with the 5 speed manual. On cold start it pulls well and drives like it's supposed to but once it gets to about 1/4 on the temp gauge it starts stuttering like it has a vacuum leak or misfire. Only problem with that is I've smoked it several times and can't find any sort of vacuum leak. No audible hiss either. Car was drug out of the woods after sitting with a failed fuel pump a couple years ago so many parts have been replaced as preventative maintenance. I'll leave a list below. Only possible issue is I had to fit my own cooling fan switch because the PCM signal wasn't getting to the fans and as a result it runs a little below the center mark. Car has also had the converter taken out and had the exhaust redone from there back. Other than that, nothing is out of the ordinary. O2 sensors are reading good and the car isn't throwing any codes.

New parts:
-Fuel pump, filter, and injectors
-MAP sensor
-Coolant temp sensor
-PCV valve
-Spark plugs, wires, coils, and ignition control module
-Idle air control valve
-Multiple rubber vacuum lines and fittings
-Both O2 sensors

I wish I could give more info but there's nothing else going on that could help point to the cause. My only solution was to see if someone more experienced with these cars could help me narrow it down because I'm completely lost as to what it could be.

 

check fuel pressure when cold and after it warms and starts being unhappy.

 

Block off the EGR valve to see if it gets better. EGR valve should only open under load and if they leak or stick it can cause a shuttering. After that you will need to determine if this is spark of fuel. A decent scan tool with track spark misfires and fuel richness. If you see an increase in miss fires your will need to watch the fuel mixture. If miss fires happen as the car gets lean and load is increased, then it is most likely a fuel issue. If miss fires just seem to happen regardless of fuel mixture then it could be something like a crank sensor and a defective ICM or even just a bad connecting to either.