Interesting thread. I have a Garmin 60Cx that does (did) the same thing...random shut-offs due to vibration when I have it mounted on my bike. I don't use mine for training but for geocaching, so lost data isn't an issue, but it *is* annoying just the same.
I bent out the battery contacts inside the battery bay without taking the case apart. Also added a thin piece of foam to the inside of the battery cover to press on the batteries and keep them in place. Seems to be working fine now.
All vintage, all the time.
Falcon Black Diamond
Gitane Tour de France
Kuwahara Sierra Grande MTB
Bianchi Super Grizzly MTB