This is on my 1987 Chevy R30.

Background -
Bought it a few years ago. TB was worn out, got a new base with bushed shafts, new TPS, IAC, MAP. Also found the wire to the O2 was severed and installed a 3 wire (heated) in place of the stock 1 wire.

Truck ran well, but always seemed low on power and I'd get an occasional CEL for lean. Upgraded both fuel pumps (2 tanks) to the EP361 (Delco brand) pumps. No more CEL, but still seemed to not pull like it should.

Did some basic data logging with Winaldl and saw consistent BLM values in the 140-150 range. Everything else looked good and it drove well so I left it alone. I did replace the distributor with a factory GM unit as the stock one was getting noisy and I found a lot of play in the shaft.

Pulled the factory TH400 and swapped in a NV4500 (overdrive...yay)

Recently I had the exhaust redone - 2.5" from the manifolds (no headers) to a single 3" muffler. BLM values after that were consistent 150s.

Installed a Performer TBI intake. Also installed a Aeromotive FPR and removed the FPR on the TBI. This was done in preparation for future mods.

The issue now is that I have a horrible surge at 1800-2200 RPM. If I roll into the throttle it starts surging horribly to where it isn't even driveable. I've been battling this for a few weeks now.

So far...
Checked TPS operation - nice smooth output, nothing funky where the surge is happening.
MAP output also looks fine.
Reset IAC.
Set idle speed so that the IAC counts are right at 20 (see below...).
Have fuel pressure set right at 12 (have tried as low as 7 and as high as 20).
Data logging shows that BLM and INT are both into the 150 range. O2 sensor voltage is down consistently under 0.05v.
Timing set at 4 degrees with wire unplugged (have gone from 0 to 10 advanced while troubleshooting with no change)

Has the intake/exhaust mods just taken the ECU beyond what it can handle?

What am I missing here? Almost seems that O2 sees lean, then it dumps in a ton of fuel, sees that its rich and pulls a ton of fuel. Exhaust smells extremely rich while this is happening. Watching the spray from the injectors I can see the increase/decrease in volume while this is happening.

Something else that I have had for a long time is a high idle. This came in before I started doing the modifications. I do not want to focus on this yet...I need to work out the surge. Just wanted to mention it idle in case they were related. It always wants to idle at about 1000rpm when warmed up. I can set idle to get about 20-25 counts on the IAC, but then the IAC counts creep up and I keep getting a high idle.

I don't have ability to burn chips. I am looking at the EBL setup from Dynamic. I just received a new 02 sensor (Delco) and was going to give that a shot, then the EBL setup is next (EBL was in my long term plans anyway) so that I can get better data logging to try to trace the issue.

What am I missing?