Car starts stuttering when pressing the gas pedal...Help please!!!!?

Ok, my car starts stuttering when pressing the gas pedal or it will die out sometimes when coming to a stop...once you get to going at a higher speed it won't stutter anymore... We changed the fuel filter and it had rust in the old one... The oreily auto parts place said that there was water in our fuel so he told us about "heet"...we put that in our car and it still does the same thing... My boyfriend checked the spark plugs and one of them (where it suppose to be white on the inside) looked like it was covered with oil or something...someone else told him that there was nothing wrong with the plug...What could be the problem and what's the cheepest easiest way to fix the problem...someone please help..i'm on maternity leave with no income at the moment and about to go back to work...so i need my car...thanks for any comments...