I had the same problem. Its Torque Converter Shudder. Go to this site try the stuff to see if its what you have.
http://www.agcoauto.com/content/plugins/p2_news/printarticle.php?p2_articleid=203
What is Torque Converter Shudder?
Driving about 45 MPH there is a sudden shutter in the vehicle. It almost feels like running over a rough spot in the road or cattle guard. As quick as it appeared, it is gone, until the next time it occurs.
Such a symptom is often a torque converter shudder. A torque converter shudder is a brief shake or shudder in the vehicle when the torque converter goes to lock up. Very often this problem is mis-diagnosed and sometimes transmissions are needlessly rebuilt as a result.
One way to identify a torque converter shudder is lightly touching the brake pedal, while carefully maintaining vehicle speed with the other foot. If the shudder immediately stops, with brake application, the problem is likely the torque converter clutch. When brake application is seen most torque converter clutches will be released. A trained transmission service technician can also test for torque converter lockup using a vehicle scan tool, specific for the application.
