I call this surging and there are a number of issues that can cause it. If it happens all the time, would suspect the throttle body's IAC valve or a sticking air flow sensor. Another cause is the coolant level is too low or has air in the system. Try bleeding the system at the two bleeders. Is there a check engine light on? Gently tap the sensor that is located directly across from the throttle linkage. That is the air flow sensor. The IAC valve is located on the throttle body and could be taken off and cleaned. Coolant runs throught this rascal.