My first guess would be that one of the switches on the brake or clutch are triggering by themselves (faulty) and disengaging the cruise control. Both switches are easy to replace. They look very similar, but I do not know if they are interchangeable. I would probably start with the brake switch, since that has likely seen more use/wear.
Clutch switch PN: 996-613-114-01
Brake switch PN: 996-613-113-02