1991 jetta idle issue?

i just bought a 1991 jetta and when it starts it idles normal for 2 or 3 seconds then jumps to 2k rpms and stays there until it warms up then it jumps from 900 to 2000 rpms up and down constantly any help would be greatly appreciated

Most idle problems are caused by the idle stabilizer, but it usually make the car die rather than accellerate. Check the idle booster switch that jumps the idle up when the A/C is on.