Cruise Control Surging
A mate fixed the same problem in his Mazda 929 (which may be worth consideration. the Maxdas had the same unit as an XF, which may be similar to your EB.
The underbonnet unit contains about 3 vacuum valves operated by solenoids. The solenoids are controlled by the cruise computer. If the car needs to go faster, the computer will send a signal to open one or more valves to apply vacuum to the servo, which pulls a cable attached to the throttle linkage. The more vacuum, the more pull, and the more acceleration.
With age, the rubber seals on the vacuum valves can perish and become sticky. So when the car needs to speed up, and a valve sticks, the computer senses that speed has not increased, and sends a signal to open the next valve. If the first valve then frees itself, heaps of vacuum is suddenly applied to the servo, and surging occurs.
The computer then shuts down all the valves as the car is going too fast, and the process starts again
The fix was to dismantle the unit, apply rubber grease to the seals on the vacuum valves. Only a real light smear was required.
hope this helps!