Cruise Control problem
 

94 S Class

When driving on the highway with the cruise on sometimes after hitting a bump the cruise will go off. When I try to engage it again it won't work,the only way I can get it to work again is to pull over shut the car off and restart.
Then it works. It also goes out if I drive over a course area in the road.
What causes this to happen? and how can I fix it?

bump for help