2007 Ford Freestyle F/W drive, intermittently bucks from a stop.

What is your question?

The bucking stops when you let off the gas, but may return when reaccelerating. It feels like it is going to die out when it happens. Could this be from water in the gas? My brother-in-law sold this vehicle to my Mom, and he thinks that the gas station that she uses has a history of that. My Mom states that as the bucking occurs, she can hear the engine slow down, then speed up as she lets off the gas pedal. As it is, this vehicle is not safe to drive. Thanking you in advance.