Just a quick follow up to this question of mine from last year.
Six months later, I have now worked out the problem.
It was just the main unit was set wrong for the transducer.
The transducer is dual frenquency, 200 and 50kHz, and the unit was set to just 200kHz. By switching it to Dual, the proble, and problem was completely resolved and I now have perfect depth readings at all speeds.
Just in case someone else had the same problem and couldn't work it out......
(By the way, I called Garmin's helpline, but they were as foxed as everyone else).