Have you replaced the IAC (idle air control) valve? That is a classic symptom of one that is bad. Is it throwing any codes? Is the check engine light on? If so, you might get the codes read and see what it is. I am strongly leaning towards the IAC valve though. However, its difficult to tell on here a lot of times with such little details and not being able to listen and look at the vehicle. Well, anyways, good luck and I hope you get it figured out.

Hi the whole subject about cars is based on one factor wear. when cars are new the metal surfaces are raw the actions of use produce wear. this wear eventually reduces the power out put so much so that by the time it reaches it's 8th birthday even if maintained with oil changes and oil filter changes the modern car engine is at the end of t's working life. hence why event extended warranty companies will no longer support further cover.

It could be a problem with the model. Try looking up the model and year it was manufactured on Google or Bing. Otherwise there is something wrong with the "coherence?" of you vehicles machanics because of trauma. Whatever it is Good luck!

Depending on which engine you have (which you didn't bother to tell us), it can be caused by a bad IAC, or a hole in the PCV hose.

check and adjust your minium idle air rate . . .

clean the throttle body and plate area . . .