Excessive idle speed

2002 Cadillac - standard NorthStar - 225K miles - on a level road, without cruise or accelerator pressure, vehicle will acellerate to and hold about 60 MPH. Shifting to Neutral allows RPM to increase and hold steady at between 2,400 and 2,600 RPM until it runs out of gas (for you guys who ask How Long?) Or until the vehicle comes to a complete stop. After complete stop, takes about 3 seconds to begin a gradual (5 seconds) reduction in idle speed to 1,400 to 1,600 RPM. Same problem occurs anytime engine is at normal operating temperature. Replaced all sensors called for in error codes. Cleaned throttle body. I am not a mechanic, but I believe this is not operator caused stupidity like a sticky accelerator. It is in the computer controlled idle (fuel) system somewhere. Two different shade tree mechanics who charge like Cadillac technicians tried but failed to fix it. THANK YOU.