1998 Ford Taurus What is wrong with my car?

Hello. Recently I have been having problems with my 1998 Taurus. I've had to replace the battery when it wouldn't start but still had problems with the car acting as if it wasn't getting enough gas. A friend, who is a car mechanic, suggested that I get the fuel filter replaced. So I did. However, it is still having problems starting and wants to die when it is idle. It also acts funny when I am driving it. It tends to speed up itself and stay at a certain mph without me touching the gas pedal. I know that one of the spark plugs are bad but I don't think it has anything to do with the way it is acting. Any advice or possibilities of things that could be wrong with it would help. Thanks!