E Class Cruise control

I have an E Class CDI 220 - There is an intermittent fault on the the
cruise control which I can't fix. Some days it works faultlessly and
other days it won't work at all. It never fails in operation.
Sometimes simply stopping, waiting and then starting againn seems to do
the trick. The only fault showing in the brain was a faulty brake
light switrch and I have replaed that to no effect. Anyone any ideas
please.

Wilby

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