Throttle Issue (Lurching)

[ryman]

I have been having a throttle problem with my 06' M Roadster ever since I bought it. While driving at a consistant speed with the rpms slightly high, over 4500, the car will just start to lurch until I let off the throttle. It seems as if the only way to prevent this from happening is to constantly vary the throttle. Luckly this doesn't happen when cruising on the freeway. I have brought it to the dealership several times but am told that there is nothing wrong.