If your Garmin 305 has become faulty and the altitude reading no longer works here is a good work-around I have found. Firstly let me say by faulty I mean the elevation shows up as something like 5,000 or 20,000m.
The work-around is really simple, instead of using the Training Centre software supplied by Garmin head over to Zone Five and grab a copy of their excellent application Sports track, best of all its free. http://www.zonefivesoftware.com/SportTracks/
Here is an example of the fix in action.
Screen Shot of a ride taken from the Garmin Training Centre software
Same ride but with the data in Sports Track.