Currently at 128,000 miles, just replaced plug wires and plugs. Also replace coil. If I had this issue for about a year now and can’t seem to pin point the problem. If hooked a smoke machine to the truck to check vacuum leaks. Cleaned throttle body many times. Checked the throttle cable. When driving when I shift gears the idle stays floating around 3k then slowly drops or just stays put. Very embarrassing sitting at a red light at 3k and people looking over.

 

Have you checked your IAC unit? Just a thought.

 

Brand new motor craft IAC and TPS

 

What about MAF sensor?