Engine vibrates when idling and surges at any speed

Hello;
I have a 96 ford explorer xlt 4wd with 300k v6 4.0L pushrod
and automatic transmission.

What It’s doing is: vibrate when idling, and surge at any speed.
When engine is warm or cold. The problem has gotten more noticeable over time.
I have changed plugs, wires, fuel filter and air filter. This has made absolutely no change.
The shop found no error codes or leeks on the intake manifold.

I know it could be a lot of things, what I’m hoping makes my problem unique
Is when I first start the engine (warm or cold) it runs at 1500rpm for 30 seconds
When it drops down then the idle gets rough. If I bring the idle back up to1500 manually
It’s still rough. I think it’s something electronic or the EGR system but don’t know where to start. Any pointers would be appreciated.
Thanks.