Have you changed the offset?
The sensitivity to kickback from bumps can be made much worse if the offset is increased greatly (to make the wheels look wider by sticking out of the arches more).
If you want this look without spending more money, you'll have to put up with it.
The real solution is to widen the rims by equal amounts each side, with the limit set when the inner wall of the tyre hits something on the chassis/suspension. This may not give you the "superwide" look you're aiming for.
Remember that any increase in tyre width will make the car more likely to tramline. This may be what you are experiencing.
You may also have to live with this, but you could try increasing the tyre pressures a bit to see if things improve.
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike