Iso Heet is also a good one to use. It has isopropyl alcohol, which absorbs water. Any water in your tank will go through the fuel system. Be sure to run the tank all the way down.
Iso Heet is in the red bottle. Don't get the Heet in the yellow bottle. That stuff is methanol, I think. It will prevent your gas line from freezing, but it doesn't absorb moisture, and hence won't extract moisture from the tank.
A classic symptom of water contamination is the engine power dropping off at high RPM. That's due to the water being stopped at the fuel fiter and preventing flow. Since your problem occurs at idle, water may not be your issue.
But hey, it is winter, and it never hurts to run a bottle of gas dryer!
2003 Ford Ranger | FX4 Level II | 5-Speed Manual
A few minor mods... nothing cool enough to mention.