Has anyone ever had this experience and could tell me what possibly needs to be done to fix it.

Car starts fine....after a few miles she begins to buck, no matter what gear Im in....usually at around 40 MPH....the newest thing I experienced today was that she won't go above 25 MPH in any gear until I shut the engine off and restart.....Whats up with that...?

I'm thinking the computer is shuting the engine down for some reason.....Has anyone out there had a simular issue at any time.