My cruise control doesn't work unless I slow right down, then push the button. It won't light up on the dash if I'm travelling at motorway speed and push the button.

Then, when I do get it to go on.....it switches itself off after a few seconds....

Any ideas? I don't have any error messages.....

I know this is an old thread, but I have the exact same problem.
Any idea if this was resolved?

Not something silly like a loose wire and slowing down helps stop it bouncing around? Ed

I had a problem with cruise control keep switching off after a few seconds, sometimes a minute or two. After some investigation I found it was a faulty (aftermarket) brake light switch. Fitted new genuine (cheaper then aftermarket! about £12) & problem solved.