BTW, what is the relay on the firewall behind the under hood fuse block. The red lead coming out of the relay is TOAST! I need to find where that goes.

If it is the same as a Caprice it is the fuel pump relay.

Keep a notebook of the codes and what repairs have been done.
Take the negative terminal off the battery to clear the codes then run the car to see if the codes come back and/or the stumble goes away.

It is not hard to check the TPS but you need a good digital meter or any analog (needle) meter to catch a "dead spot".
With a little care it is not hard to back probe the TPS.
If the TPS is normal the PCM checks and sets throttle closed voltage every key cycle.

Check this active thread for some ideas:

https://www.impalassforum.com/threads/strange-fuel-pressure-numbers.1324989/?post_id=12195387#post-12195387

Charm has some wiring and mechanical information:

Free Car Service Manuals from Operation CHARM — No strings attached!

Operation CHARM provides completely free repair/service/workshop manuals for over 50,000 models of cars and trucks manufactured 1982-2013. No sign-up or paywall.

charm.li

It is much easier and in some cases mandatory to have live data from these cars.
I always recommend a live data scanner or Eehack.
Sometimes you need a oscilloscope.

 

Keep a notebook of the codes and what repairs have been done.
Take the negative terminal off the battery to clear the codes then run the car to see if the codes come back and/or the stumble goes away.

Since I have been using the CC screen as my diagnostic tool I have been using it to clear the DTCs. Yesterday I disconnected the battery when I was fiddling with the electric window switches. This morning I went for a drive. Like I said in a previous post, at cold start up and first drive the car runs great, no stutter or stumbling. Once it goes into closed loop it begins the stuttering and bucking, especially noticeable at a normal steady cruise speed. Today I tried something different when it started acting stupid, I disconnected the vacuum hose from the EGR valve. Instantly took care of the trouble I was having. After a 30 minute drive I checked the DTCs on the PCM and came up with 00. Car is super responsive(peel out mode) and runs nicely. No codes showing up, not exactly sure what I need to do about the EGR system. I've been riding with the vacuum line disconnected and plugged.

On another note, the CHARM site is fantastic.

 

Thanks for the reply and TPS tips and the heads up on the active FP thread.
EDIT: Is that AutoXRay 3000 something I should consider since I don't have a laptop handy?
I found the female end of the chewed red wire on the inner fender when I first started working on the car a few weeks ago. My '94 9C1 doesn't have the FP relay on that stud, there is a ground strap on it.

Funny when you mentioned CHARM I was actually already on that page diagnosing DTC 18. I'm thinking I'll find some more open wiring before this is over with.
Haven't had time to do anything on the engine diagnostics since I posted. I spent most of the evening getting the windows going up and down again with new window clips.
This is going to be a nice sled when I get it ironed out.

 

Yes, you absolutely need a scanner to monitor live data so you can see what's sending bad data to the PCM. I LOVE EEHack - makes diagnosis a breeze. Go find a cheap used Dell business laptop on Ebay and buy the cable and you're good to go. More guidance here.

 

Your next step is to get EEHack or a scan tool and monitor the commanded EGR duty cycle. See if the PCM is commanding it once it goes into closed loop.

On a side note, I've seen on more than one occasion someone has directly connected a vacuum line to the EGR valve. At cold start high idle, the symptom is masked, but once the car warms up, you get a definite rough idle.

Finally, for anyone reading this and looking to disable EGR, it is important to disconnect the EGR solenoid electrical connector, or disable EGR in the PCM calibration, otherwise the PCM will try to enable it, add 5 degrees of spark advance, and the rod bearings will suffer.