controls suddenly jam and the magically free themselves just before impact (sic)
Mr Purdey,
Have you ever read the account (originally in their Flight Safety periodical) of the US Army Chinook on a post-maintenance delivery flight which went into an Uncommanded Flight Control Manoeuvre from something like 5000 ft agl? The one where the aircraft inverted, went into a Vne+ dive then recovered just above the trees, while not responding to aircrew control inputs? If not, you might find it enlightening. You might then not be so quick to dismiss this as a
possible cause. I have to stress
possible, because we'll never know for certain, will we? If you've not read the account, then please acknowledge here and we'll see if there's a contributor out there somewhere who can lay his/her hands on the article.