I've just posted my Garmin 305 back to their HQ for replacement because it was turning off while I was using it. Apparently the battery is linked to the main unit using something like a spring and sometimes this can get knocked out of contact and the unit powers down. It's worth emailing them if firmware updates don't work (they ask you to do that before accepting a return).
What is the best app or site (on Mac here) to look at your data after rides??
I'm using motionbased.com and a third party program called Ascent (costs $39 I think) both are pretty fun and have a range of stats etc.