What year and which engine might help us to understand your problem.
On a V6 with manual trans, especially the SVT, there is sometime a little bucking or surge with closed throttle or nearly closed throttle in 4th or 5th gear at about 25 to 40 MPH.
On those it helps to confirm the condition of the plugs and wires and then clean the throttle body and injectors. Often cleaning the throttle body and injectors will make a big difference. If it persists, do the throttle hang fix described in the FAQ section. Sometimes this is caused by the PCM trying to lift the idle speed while you are coasting or nearly coasting. The throttle hang fix settles the idle down.
You may have other problems, but if it acts like I described, it is worth trying.
