I'm not sure, but I had a similar problem and that's what it was. In my case, I had my ALDL connector for data logging connected. To connect to my particular computer, I need to use a 10k resistor. If I don't disconnect the resistor after connecting to my ECU it will idle at 1000 rpm.
I went through a lot of the same steps. I reset base timing, reset IAC so it was at about 15 counts, and set my TPS to .55 volts. After I hooked my data logger back up, it idled at 1000 rpm again and I lost my shit. I tore through every hose and nook and cranny and found a small vacuum leak. I thought I was triumphant having fixed it only for it to continue to idle at 1000 rpm after I started it back up as if to give me the middle finger. That went on for a few days. At one point I had started it and then hooked my data logger up and noticed a jump in rpm. TBIs kind of idle high from cold anyway but I thought it was weird. Once it was warm i disconnected the cable and the idle didn't come down so I shut it off and restarted it to see if it would jump again when i reconnected it. It started to idle normally for the first time. Then I connected it again, and right back up to 1000 rpm.
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