I suddenly have the problem of rough idling when I come to a stop. If I stop and put the car in neutral, it idles smoothly.

The only thing I have done recently I topped up the crankcase with oil. Other than that, not even a recent fillup with gas.

It did appear to start after I got off the freeway in which there was an especially heavy rain/snow shower.

I am new to this car, so I would encourage suggestions of even the most simple thing.