I removed and cleaned the TB and replaced the gasket just in case.
I also checked the air filter and it's good to good (gtg).

 

Sparkling clean, y’all didn’t move the butterfly plate manually did you?

 

Not since the rag incident months ago.
Is it possible for a bad fuel rail to cause an issue? Fuel cleaner makes it run smoother and it was running really rough until I checked the fuel pressure at the rail and it started using better afterwards.

 

You can remove the fuel rail and clean it out.

 

The engine still shutters once warm.
When cold, it seems to run very smooth.
Thoughts?

I rechecked the fuel pressure at the rail.

With the key left in the accessory position and the engine of the pressure went from ~58 PSI to 40 PSI in 30 seconds.

It was 60 PSI at idle, dropped to 56 PSI the moment the key was disconnected and had dropped to 54 PSI 1-minute after the key was removed from the ignition. Is this normal? The ACDelco fuel pump only has ~1K miles on it.

 

That sounds close to right. It is supposed to be measured key ON engine OFF with pump commanded on (with a Tech2). 50-60 PSI.

Turn pump off

Allowed to lose 5PSI in 1 minute. Then reduce to 10 PSI then allowed to fall 2 PSI in 5 minutes.

The fuel pump only runs for 2 seconds engine off.