00 2.4 cav idle problem 1500rpm to 3000rpm

2000 cavalier z24 2.4l 5 speed 140,000k
Hey guys i have a serious problem, It all started about 2 weeks ago.
When i start the car it would idle at 2000rpm and when i drive it
would also idle at 2000rpm but when i stop after driving it would spike
to 3000rpm and would go back down to 2000rpm only after 20 seconds.
Also while im in gear driving at 60kh and let of the gas it doesent go down
in speed almost like cruise control is on until i apply the clutch or brake.
So i looked for vaccum leaks and non were found but after i changed the
o-rings on the fuel injectors and spark plugs/boots the problem has been
fixed but now after a week problem came back. so i went to the scrap yard
got an intake manifold with throttle body and with all the sensors still on.
changed it problem still there. Changed fuel filter nothing. My guess its one
of the sensors but i dont know where to start the computer is not throwing
any codes eather.
Any suggestions ?