So I have a 1990 5.0. When first started it will hang at like 1500 2000 rpms then slowly go down to 1000 and stay there. But if I tap the throttle it comes down to 600 the. Starts going from like 400 to 1000. Just back and furth. It's surging or something. I already have done tps and iac sensors. Any help? Thanks in advanced.

 

Sounds like a vacuum leak.

Check and or replace those brittle nylon lines with silicone or rubber vacuum lines.

If that checks out well, inspect both vacuum canisters. One on the passenger side and the other is located under the coolant/washer fluid reservoir. They tend to rust out underneath.

Next is the white vacuum line located on the Passenger side that runs to the vacuum modular for the blend door. It's widely known it becomes brittle and breaks and makes for a huge vacuum leak.

Next if all the other ideas check out or are resolved, is the throttle body. I will say this... If tapping the accelerator brings the Idle down, then it sounds like the bores of the throttle body are worn, which is due to the blades wearing the bores down.

Post back with your results.

 

I am with TS most likely a vacuum leak from a line but think EGR valve also the pintel inside the egr may have carbon or the seat may have carbon build-up.If no vacuum leaks are found from lines buy a gasket for the egr and clean it.