I've had this problem with my old MK3 unit and now the MK4 is playing up.
Basically, it struggles to boot some mornings. It loops in an almost endless reboot cycle, only usually making it as far as the logo screen, occasionally showing the menu or map for a few seconds and then rebooting again.

Strangely, some mornings it will work just fine. I've noticed that it *might* be related to colder weather.

If I go on a long journey (an hour or more) it may finally start working around the end of the journey - warmed up perhaps?