The most common driveability issue on your MY is a flakey DPFE sensor and it will often show up while cruising at or around 40 MPH. The DPFE is the monitor of the exhaust gas recirculation system. The EGR system takes exhaust gasses and inputs them to the throttle body or air intake. Carefully metered under specific circumstances the exhaust gasses cool the combustion chamber temperature, reduce tendency for pinging and preignition and reduce emmissions.
The computer commands EGR by signalling the EGR controller to apply vacuum to the EGR valve to open the valve a specific amount for a specific period or pulse. The DPFE provides feedback to the computer to tell the computer the rate of flow of exhaust gasses.
If the DPFE is not measuring right the computer may command even more EGR when it already has enough. This can interfere with the air fuel mixture, cause misfires and contrary signals will be sent to the computer by O2 sensors.
The result will be some bucking, chugging and surging. The computer with bad feedback may also start to "hunt" for the right gear and amount of torque converter "slip" to match the TPS position, interpreted load and power available.http://www.ehow.com/how_7970844_test-ford-dpfe.html