Garmin Edge 305 shutting down

I have a problem with my Edge 305 - it shuts down at a random basis
during
rides - altho it most often happens during fast descent. It also stops
the timer
when you do hard stop - eg. if stoplights caught you by surprise - and
it does
not restart it. It is very annoying especially if you try to beat your
previous best
time and suddenly you realize that timer is stopped or unit is off.
Are those common problems with Edge 305?
Is 705 any better in that respect?
(I bought 305 at REI so I can always return it and swap for sth else -
eg. 705).

TIA for any info/pointers,

JT