Need some help with bad idle

[ZeRo2o9]

I have a 1990 5.7 tbi with 225k miles and it runs fine when accelerating but when I’m idling in park or sitting at a stoplight I get a random shake where voltage and rpm drops for a second and goes back to normal, it’s random it can happen once or a bunch of times on my way home from work. I was going through everything and did a tune up new ac delco plugs the cr43ts, wires cap & rotor from rock auto tune up kit united auto, new tps iac cts and pigtails for them, new ignition coil, tbi was rebuilt last year fuel pump was replaced almost 2 years ago, all parts were either ac delco or parts store brand if I couldn’t get it. I had a high idle issue at one point and found out the pigtail for iac was bad and I just went replacing old parts I thought were original. I literally have no idea what it could be I have some knowledge but I am not a mechanic and do not have the tools to read data from odb1. Is there anything that could cause what seems like a misfire at idle that I haven’t already replaced? My idle is smooth at looks like 600 rpm or maybe right on the bottom of the line for 600.

 

[Schurkey]

There's all kinds of things that could cause misfire problems.

If you don't have diagnostic tools, you'll replace stuff at random, and someday you'll replace the right part(s) or you'll get sick of the process and sell or scrap the truck.

Remember that you can replace "parts" all day long and twice on weekends, and it won't do a bit of good if the actual problem is in the wire harness or connectors.

You NEED to communicate with the computer to find out what it's wanting to tell you.

And you should verify fuel pressure at idle and under load.

I'd be looking for vacuum leaks, defective parts including the stuff you've already replaced, potential intermittent issues with the ignition, including a broken magnet on the mainshaft. (Common TBI problem.) But there's other stuff as well. Loose battery cables, failing electrical connectors, and much more.