First, check for any firmware updates that may contain a bug fix.
To check for firmware updates for your Garmin Head unit, you can use Garmin WebUpdater or connect to your computer via the Garmin Express software.
If an update did not help, and you have already fitted fresh batteries as well, then please make sure that the battery cover is properly installed; if that's not the problem then try testing the batteries with a voltmeter to check if they are indeed good.
I don't see in your messages where you already tried checking for firmware updates?
I also don't see where you said that you checked the battery cover is properly installed?