I've been testing my Forerunner 305 and so far it works 100%. I've only downloaded a few workouts, though.
Are you using the Heart Rate strap and/or cadence sensor?
Anything different you might be doing? I just turn it on, hit start, then stop and reset before downloading.
BTW - the tracks (at 1 point per second) look AWESOME so far. I can see I'm going to have to go re-GPS nearly every trail around because this data is so good (very smooth turns).
Thanks.