The OEM Nuvi mount is meant to hang off a car windshield, not very heavy duty (the Zumo pictured on the bottom is a different animal altogether).
If you're going to be sticking it on a vibey, wind-laden set of handlebars you should really go with something like this to at least hold the unit - then attach a ram ball to the back and tie to an arm or shortlink to a ubolt on the bars.
RAM Mount System
The RAM solution isn't very 'pretty' (well, they do sell Chrome parts now for the harley crowd
) but it does do the job fairly well.
Quality is expensive but you only need to buy it once...