Driving home today on the motorway with the cruise control switched on, I noticed that the cruise switched itself off - it was still indicating on, but seemed to be as if I'd hit the cancel button (which I hadn't) so when I pressed the resume button, it got back up to the set speed.

After further investigation, it seems to be 2 things that cause it:-
1. When going up a sudden incline (the sort where the suspension is compressed as the incline starts)
2. When indicating right.

I have possibly isolated number 2 back to a fault with the right rear light as when I got home, I turned the right indicator on and can see the tail light getting slightly brighter/dimmer in time with the indicator (possible bad earth?). I'll check this tomorrow, but has anyone got any ideas about this otherwise?

 

Well, looks like the dodgy earth is the problem as far as indicating right cancelling the cruise is concerned. I presume that cruise must monitor any power going through the brake light circuit. The earth pin was quite rusty, so may get a new light board and plug for the long term.