My local Dealer suggested i did a reset on both helmet and zumo...bingo perfect ever since.
I'm not sure the zumo reset is in the book. The Dealer did it for me whilst telling me Garmin told him how to do it. Might be worth you checking it out but it was something like: whilst turning on you touch (constantly) bottom right of the screen till you get a beep.
Sounds about right:
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From the Garmin Website
How do I perform a Master Reset on a zumo?
01/13/2011
The Master Reset can be used to clear out any corrupt data that could cause the device not to function properly.
Note: A master reset will erase all waypoints of favorites and will restore your device to the default factory settings. If you would like to back up your favorites you can find instructions to do so here.
To do a Master Reset:
Remove device from any type of power source
Power the device Off
Place and hold your finger in the bottom right corner of the touchscreen
Power the device On
Release the power button after the device powers On
Continue to hold your finger on the touch screen until you see "Do you really want to erase all user data?"
Release your finger from the touch screen
Touch Yes
Follow the on screen prompts
*Please note if you have a Zumo 220 you will need to press the dot to calibrate the screen before you get the message in step 6.
After the Zumo is set up, if indoors, please go outside to allow the device to acquire the satellite information. Ensure that the device has a clear line-of-sight with the sky.
Links referenced in this article:
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Grey Beard