Thats electrical.

Assuming the TPS is set correctly...and the TPS could cause this...
The IAC is fine. IAC sets itself. Its a myth that each new one has to be manually set. Thats what the IAC does for a living...it finds where to be by way of instructions (steps) from the ECM. The TPS is part of that process, so make damn sure the TPS is right. The ECM will see the throttle closed, rpm of 900 and it will tell the IAC to close more until it sees rpm of 650 or so at idle...simple...BUT the TPS has to be right so the ECM knows where the throttle actually is.
IF the TPS is set and t he IAC is actually functional, (do the test described before to test IAC function)...then,

either harness issues (VERY VERY COMMON) or the fuel injectors. If you still have fuel injectors from the factory, the originals, you WILL NOT ever get it to run right until you replace those...NO MATTER the miles.

Have you gone thru the harness grounds as suggested for ALL mystery symptoms? Very important. Your car also has a jumper post...power source connections. Clean that. Most important is the ground bundle on the block by the filter.

The original inj fail due to alcohol blends destroying the seals inside and clogging the nozzels. They stop atomizing fuel and start spraying wet gas which does not burn well. The spray even stops at times...surges, poor revving & accelleration.

As previosuly stated...Call Jon.

There should NOT be a C4 from before 1992 with the stock injectors still installed. Even the mid 90s...the stock injectors were trash.

 

It's not vacuum, that's for sure. Did you clean the tb? I would clean the IAC housing inside to be sure there is nothing in there, (I'll bet it is all carboned up) Once you get that cleaned out I think I would run a can of seafoam through the gas tank to see if it makes any difference. If it improves things, then a new set of injectors is most likely needed. You could also check the ohms readings on each injector, just take a digital ohmmeter and touch the two spade terminals on each injector & let us know what they are reading.

 

Here's a pic that Agent 86 posted for me when I was trying to figure out my HVAC vacuum connections (destroyed in a fire). The HVAC vacuum line is behind the pink wire and goes into the wire loom and then through the firewall (on the left side):

 

The TPS is correctly set to .541v. I haven't cleaned the TB, but it didnt look that dirty, nor do I own TB cleaner (yet). Is there a recommended brand of injectors that I should go with?

Edit 2: Who is this Jon guy everyone says I need to call?

 

I had a similar sounding problem with my 1985 C4 (owned since new). After years of diagnosing and patiently checking each vacuum hose. I found the fuel vapor hose at the gas tank, which is the vent going to the evap canister, had big hole in it. It rubs against the body back there and is notorious on all 80's vintage Corvettes, Camaros and Firebirds for causing this vacuum leak. This is the tube (a short rubber hose) that connects the gas tank vent to the metal fuel vapor line going to the front of the vehicle. Remove the rubber fuel filler boot and took a look. Then please tell me if this solved it.

 

you do know this thread is almost 3 years old? if fact, the OP hasn't been active here in about 18 months - most likely, he's gone, and his problem was solved one way or another. anyway, thanks for the input -