Nissan Repair: Maxima 2000-High Idle RPM, engine management computer, air control valve

Question
This is very strange. My dealer has tried to fix it over the last 2 years but nothing has worked.

In idle or while driving, the RPM jumps all of the sudden to 1300-1600 RPM and it does not slow down. While driving the RPM stays at its requested value by the gas paddle even if I'm releasing it. It comes down very slow. Some days it is OK but 85% of the time it runs into this scenario.
I have recently noticed that if I'm shutting off the engine and then restart it (after 5sec) the idle RPM becomes normal (700 RPM). It looks like the engine management computer resets a accumulated value that later on will ask again for high RPM. I did have 30k ago a tune-up and the battery is fine.
The dealer has used the high pressure flush technique, changed the ignition valves (all 6) but no major effects.
I don't know what to do anymore. They say that they can't seeing anything on the diagnostic registers. It makes the gas mileage bad plus I'm overloading the brakes.

Answer
Marius,

There is a dampner that lets the idle down slowly on the 2000 model and I would check that first.  Other things to check are a gummed up throttle body or a frayed throttle cable.  There is no idle stepper motor and the idle is set with the idle air control valve so I think it is a gummed up throttle body.