Try to describe more of the symptoms. Does it only occur only when the vehicle is at partial throttle? It is more of a hesitating/surging feeling at partial throttle? Does the problem go away when the gas pedal is to the floor?

If it is more of a part throttle stumble/surge/hesitation, chances are it is the DPFE sensor gone bad. Good information on this simple fix can be found here:
http://www.focushacks.com/index.php?modid=80