High RPM on 2000 Elantra

I have 2000 Elantra. After sitting approximately three hours engine will
always rev up to 3000 RPM and climbing. I always press lightly gas
pedal, increasing RPM even higher, forcing the engine computer to
"reconsider" idling RPM. Normally, RPM is around 1500 and dropping to
500 when warm.
It appears to me as a software bug. Is there any update for this?
Mechanics could not find anything wrong with the engine. (2L engine and
manual, no air or nothing). And it is hard to repeat as it needs exact
situation. If conditions are slightly different, everything is OK.

 

This sounds like perhaps a vacuum leak or throttle position sensor issue.
In many cases, the throttle position sensor will leave a trouble code but
not set the check engine lamp.

The most frequent vacuum leak is in the PCV hose.

 

The mechanic checked out already once and find out nothing. Again, it
happens only under very special condition. Perhaps occur only once every
three months. And problem is there since the new.