I have an intermittent issue on an iMac. I have a Garmin Forerunner 305 and for the vast majority of the time my data will not upload at the first attempt. It often takes 4 or 5 attempts with me having to "force quit" Safari, and then eventually it will work okay even though the process I follow hasn't changed.
In summary, what happens is that Garmin Connect gets to the "Reading activities" screen, the bar goes to 100% complete and it then hangs.
Any ideas?