The SLK throttle is drive by wire. The accelerator pedal is a potentiometer and the throttle is a servo motor run by the ECU. Cruise control is integrated with the engine control unit. It is basically software in the ECU.
The sensors needed for cruise control are the same needed to run the engine - accelerator pedal position, crank angle sensor (RPM) - and the ABS - wheel speed sensor. If they are faulty you'd likely get an indicator light and a code.
I'd check the cruise switch and connections and brake and clutch (if you have one) switches. An intermittent open or short in one of these could cause the problem you describe.
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