he symptoms were that the zumo would take a very long to start up (say 5 min) but then would function normally. It could also start normally, but freeze after starting and then resume normal operation later. Garmin Support suggested carrying out a factory reset and this did seem to work but the problem came back after a couple of weeks. The unit has been in use for about 3 years.
Looking around the unit's filestore I found the following file:
E:\GPX\GPX\Archive\1.gpx
This file appears to ba an archive of all the tracks ever recorded by the zumo, except possibly the last 10 or so, and it is large for a text file at around 84MB.
Incidentally, this could provide a way of getting at old tracks by copying the GPX file and importing it into BaseCamp. This only works for small archives and in this case the file must be edited to remove most of the tracks not required before it can be imported into BaseCamp. The zumo 220 does not provide any track management in the unit which makes it difficult to preserve tracks when there is no access to BaseCamp on a PC.
This large file could be responsible for the freezes, since it takes some time to copy the file, and it could take the zumo a similar time to update the archive with the latest tracks. So reducing the size of the file could reduce the freezes.
Garmin Support did not recommend this, but I copied the file to my PC and deleted it on the Garmin. It seems to have worked, in that the unit has not frozen up since and has eventually recreated a much smaller version of the file with just the recent tracks in it. If you try this make sure you keep a backup copy of the original file just in case it does not work as expected.