I've had this problem, as have a few other here. Firstly, check which version of the system firmware you have - current version should be 56.00.230, I believe (not what most of them were shipped with). You'll find that on the Settings display, System tab.
That being said, that hasn't cured the problem entirely on mine and it still drops out periodically. Sometimes it turns into a compass screen and tells me that no sat nav is installed - Mazda are still investigating this (plus some other issues with the wi-fi hotspot system).
What worries me most is that the dealer has told you to wipe the card?
As far as I know, the cards are unique to each car/system and can't be cloned/duplicated, etc. Did you back the system up using Mazda's online toolbox first?
Alan