RPM drop and immediate recovery?

I don't think the yellow ABS warning light staying on is related. It sounds like there is a vacuum leak at the throttle body base gasket or the IAC control motor needs to be replaced and the passages cleaned with carb cleaner. Here is a guide you can use when the engine is cold only:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Check out the images (below). Please let us know what happens.

Used carb cleaner, did not detect any vacuum leaks. Did find a line going into side of motor (on left side facing) which pulled out very easily and was very dirty (see pictures). Not sure if this is PCV or if I should try and clean it. I thought PCV was on the other side. Removed IAC, it was pretty gunked up. Used alcohol pad and very gently cleaned exposed spring did not attempt to clean down inside of it. This did seem to make some difference. Ordered new IAC but will not be in till next week. Please forgive me for not knowing the proper names of engine parts, between the fact that the last time I worked on my own vehicle they still had points and condensers and a stroke messed with my memory.

When replacing IAC, not sure how to clean passages with carb cleaner.

No problem, which is the PCV which I would get a new one for. Here is a guide that shows how to use carb cleaner to clean the passages:

https://www.2carpros.com/articles/idle-air-control-valve-service

Please go over this guide and report back.

Thanks Ken, video was extremely helpful. Once new IAC arrives ( next Thursday) I will clean throttle port & IAC ports. Replace PCV and IAC and say a little prayer. Will reply back once job is done. Thanks a bunch.

Good to hear please let me know what happens.

Ken, changed IAC and PVC. Cleaned ports the best I could but the air flow and air intake ( pellen?) Is all one piece. Only way to clean backside is to take whole intake off. Sprayed carb cleaner in hole that IAC goes in and what I could reach at air intake. Wiped out best I could. Oftentimes when I buy parts at parts store using vin number, what they give me doesn't match what's on truck. I'm wondering if previous owner put different motor in. Can you look at video and tell if it's a 4.3 vortec vin W. Manufacture date is 5/94. Other video shows tac so you can get a better idea of what it's doing. Engine had almost reach normal operating temperature when shot video.

Thanks for the videos, they help, I would do a full tune up with new spark plugs, also I would service the throttle bore, the engine seems to be running okay, Also I would install a new MAP sensor. The engine looks stock except this is the high output engine. Make sure you install AC Delco spark plugs, wires cap and rotor. Also, I would check for vacuum leaks use this guide when the engine is cold only:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

and

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Please go over these guides and report back.

Ken, I've really messed something up bad. I put a new TPS sensor on it and now it tacs way up before shifting into 2nd and doesn't shift past second at all. It WAS shifting well except when in overdrive when you would come to a hill and start pressing on accelerator it seemed unresponsive until you pressed hard enough to drop back a gear. Now the shifting is acting up and I've got a check engine light that wasn't on before. I took new TPS off and put old one back on but it's doing same thing as new one, which it wasn't doing before I changed it.

Got code 22 using wire and check engine light. Looked It up and says weak signal from TPS. I was talking about checking voltage but I didn't understand what they were talking about.

Okay, Ken, here's what I found out: using voltmeter at TPS. When connected to beige wire shows constant 5.7 volts. When connected to blue wire shows 2.6 volts with throttle closed, as I open throttle volts go down instead of up. It's does exact same thing whether the old TPS or new TPS is on. As I understand it, the 5 volts on beige wire is about right. Blue wire should start at 0.5 & go up to 5 as I open accelerator. I don't understand what happened. All I did was unplug TPS, put the new one on, plugged it back in. When it acted funny, I just put the old one back on, and it started acting funny too. What happened?

Ken, got more detailed, accurate info on voltage at TPS.
When connected to TPS beige wire (supply) shows constant 5.06 volts (Not 5.7 as I stated before). Blue wire shows 0.22 volts with throttle fully closed, and 0.03 volts with throttle fully open.
With wire disconnected beige wire shows constant 5.04 volts. Blue wire shows constant 0.03 volts.
After disconnecting and reconnecting the battery, check engine light will go off until I press accelerator, then the light comes back on.
I also heard a sound from behind heater control, I'm almost positive it's a vacuum leak. That's an easy fix. If I could just get it back to where it was before I messed with TPS I'm pretty sure I can change MAP sensor (assuming it's just a matter of unplugging, taking old one off, putting new one on, and plugging back in) snd doing tune-up like you suggested.

Okay, it sounds like you may have a bad connection at the TPS or the ECM is going out. Double check the ECM grounds and power inputs.

Okay, I'll let you know what I find.

My feeble brain finally figured out that I was getting the wrong TPS. The one I was getting turned clockwise but the one I needed turns counterclockwise. Finally got the right one and is back running fairly decent. Still going to do tune up, like you suggested, and fix vacuum leak. With a little luck that should do it. I appreciate all your help. Thanks Ken.

Good to hear, please use 2CarPros anytime we are here to help.

How do I put money on your go fund me account? You have certainly been a great help!!

That would be great! Please go here:

https://gofund.me/c7d318ad