Drew55 - well I did not return 305 to REI yet - the thought of having to strip all the stuff (mounts, magnets, cadence sensor) off my bike and then spend another hour or two attaching replacement... But I notice that if I wedge bike light fairly close to Edge 305 - so that it pushes against it and prevents it from moving freely and (hopefully) rattling - then I have  the switch off problem more or less under control. I will investigate attaching some rubber (old broken "Livestrong"bracelet would do) to 305 to minimize it's ability to rattle if I do not have bike light on handlebars.

Woland,

Would probably be best not working with REI, but go directly to GARMIN, describe the symptom and see what they say, Once I did, it tool about 10 days for them to get me a refurbushed model and it's not shut down again since. Prior to swapping it out, the only option i had for getting an accurate reading was leaving the display unit in a pocket just pulling it out ot get a reading then return it. If it was mounted on the handlebars, it would usually remain on until i hit the first bump of any type then shut down. Plus I only had to return the basic unit, not the rest of it. all the mounting brackets, etc remained on my bike