grab the dish near where the actuator mounts to the back of the dish, try pushing on the dish and watch the shaft that comes out of the actuator and would normally move the dish, this shaft should be solid and not let the dish move while you are pushing on it if the shaft slides in or out, then the nut inside the actuator is probably worn out, these are usually made of a soft material and are not very hard to replace, if you can find one. also make sure that the actuator is not sliding in its mounting clamps
one other thing, General Instrument made a "C" band receiver that had this problem and was cured by replacing an 8 pin memory IC