2001 Ford Taurus car jerks going down the road while you ar

i had this problem about9months ago, some one told me that i might have water in my gas, i put in the stuff that takes water out of your gas in the tank,every time i put gas in the car, and it quite, it started up again last week,, it like you going down the road and something is trying to push you backwards, but the car keeps going, please help me thank you alvin