This is my second 305--the first had the well-known loose battery problem, and would spontaneously shut off in mid-ride. Though long out of warranty Garmin replaced it with a refurbished unit (yay, Garmin), which works reliably and perfectly.
EXCEPT: I have the until set up so that one display window shows cumulative altitude gain. The display routinely shows 15% or so less altitude gain then is actually the case--no matter how long or hilly the ride. When I download the data to my desktop the correct altitude is transferred!
I find this baffling and annoying. Any ideas? No one I've talked to has ever heard of such a thing before.