Will anyone ever get to the bottom of this. I hate to say it, but I doubt it.
Cell-phone/GPS interference from the punters in the rear is what I think it was caused by.
I was airborne from Leuchars to Kinloss at the time and the weather was not at all brilliant for low-flying especially at top Chinook speed. It did after all hit the Mull going balls-out. Why?
I flew with them both in the other seat in the 80's and they were both good blokes. It certainly was not pilot error unless they were drunk. (And if you believe that then my cockerel IS a kipper).
Sadly, there are demons about.