That's interesting. Prior to the weekend I had the HCx mounted on an Acor bar extender tightly mounted to the bar. The bracket was an oversize one padded out to fit on the normal size Acor. So a little bit of give, but not much. Never switched itself off due to road shock despite some hard hits.
Prior to Saturday's Swindon ride I thought I'd fix somthing that wasn't broke, and refitted the Acor with a couple of strips of rubber at the bar clamp. So there was a bit of give there and some movement when going over bumps, which I hoped would have a shock absorbing effect.
Not so. For the first time, on Saturday, it switched itself off 3 times, twice over cattle grids at a fair speed. Over bumpy sections the device appearred to be vibrating quite a lot.
It seems it is better, unless you have some way of damping the vibration, that a more solid mounting is better, which ties in with your experience.
I shall update in due course.