Is HEET something that would make a difference with this? It's weird that exhaust sounds bad, which was the first symptom that I noticed, but I guess related to the idle.
If you think it is water in the gas, Heet is a cheap fix that may help you out as long as there is more gas then there is water in your tank. I put it in and let it sit over night, it mixes with the gas and absorbs the water.