Thanks for your advice, RaTTuS. Sadly this does not have any effect in Raspbian (or the old Debian Squeeze image, either).
The USB hub inside the Lapdock seems to be disconnecting of its own accord, then reconnecting. It's like the plug is being pulled then reinserted every 3 seconds.
I think this may be an "Elephant" problem. What a shame, the Lapdock is such a lovely integrated solution.... have to stop using it now.
Unplugging physically after testing it is even worse. Gazillions of errors, and cannot ^C out of the tail command. Pull the power plug is only solution. Imagine if I had a HDD plugged in! Boom.