1996 Chevy Corsica Car rocks back and forth after I start I

I just recently took my car in for a tune up and they replaced the spark plugs, plug wires fuel filter and a serp belt. The day after when I started my car everything seemed fine until my car started making like a reving up sound and rocking back and forth while it was still in park. It also dose it when I am in drive or reverse but not moving if I push on the gas the car will jerk as it goes and when I let go of the gas it will keep accilerating for like 3 sec or so after I let off the gas. Then it just stops rocking and runs just fine. It dosn't do this everytime I start up but most of the time. I took it back to the place that did my tune up and they think it could be a air pocket in the coolent. I don't know. Help