I had the same problem and tried the above with a similar audio noise filter but to no avail.
Currently, the cruise control will either surge say 20km/h or more so you have to disengage it via the brake, and/or it will just not engage again. When opening the cruise control controller box, the magnet connected to the servo is disconnected from the magnet controlling the throttle cable. In the reset position, they both touch and the throttle cable has the throttle plate in a closed position. Sometimes, when turning the car back on, the servo will reset and it will start working again, but this resetting is intermittent, it doesn't occur every time you turn the car on and off after the cruise control stops working.
Via dip switches on the cruise control, it is currently set to '5 or 6 cylinder' when my car is a 4 cylinder. I'm going to change that and see if it changes anything. It's worth saying that when the cruise control is actually working, it works well, this is why I never changed this setting previously.
Also I don't understand / don't know what to set:
Pulses Per Mile - 2000, 4000, 5000, 8000.
Control Switch - Open Circuit Control Switch, Closed Circuit Control Switch.
Does anyone know what they mean / how I determine what is the correct setting for my car?