98 Cavalier z24

I have a 98 z24 cavalier when I start it it will run good then will drop down to 500 rpm then up to 1300 rpm it will do this 3 times then run good if u try to drive it, it will have no power to take off when this clears up it runs good this will happen if it is warm or cold
i have put in a new computer t.P.S - m.A.P - and a air idle sensors what else could cause this