98 olds intrigue, shakes after ignition.?

I have a 98 Oldsmobile Intrigue with the 3.8L V6 it has almost 150,000mi on it. It runs smooth for the most part. Occasionally though it idles pretty shaky after starting the engine and I noticed the RPMs jumping as it's doing this. After I putting the car into gear and driving the shakyness quickly ceases. What could be causing it to do this? I've checked the spark plugs and wires and they seem to be in good condition except for running a little too hot, and the timing belt appears to be in excellent condition. A couple of other things I notice though is that the oil level seems to deplete gradually between oil changes, but I have not seen any oil leaks anywhere. The exaust seems pretty normal too, no smoke or anything. I'm certain it's not the motor mounts either.