and the latest news from Garmin is:
Having replaced the bushings which they sent me and alas had no effect- Garmin now tell me they now have a 'new' version of the Montana 700 with altered electronic bits and bobs to correct the shut down problem and they are replacing my duff one with one of those under warranty.
Bliss
p.s.
new version of GE is out V7.22.0.0 today