It's often leaks in your intake manifold which dominate the air-fuel mixture at idle (not at speed) and cause the engine to rev up and down - too little to see on the tach, but detectable on the oil pressure gauge and by ear.

As me how I know.

An excellent way of detecting leaks is to take the car to your indy and have him pump inert white gas into the engine - intake leaks are pinpointed with this technique.