GME G-Plot G142C keeps losing map data

[Sustainablecam]

Hi folks,
I have a GME G-Plot G142C on my boat and I find that for no apparent reason it keeps resetting and losing map data (and then just gives me an essentially white screen with boat icon tracking a line behind it and showing the GPS coords).

Alternatively sometimes when I turn it on it comes straight up with the white screen and never shows the map at all.

I've googled and youtubed to no avail. The owners manual is completely unhelpful.

Sometimes a message will come up and say that the memory card is full (presumably from previous owner making lots of waypoints etc? I have no idea.) Could this be linked to the main problem?

Any suggestions? Thanks in advance!