My SAT NAV has recently started to exibit a problem where it "forgets" it has a DVD installed. Popping the DVD put and in appears to fix it for a while then it drops out again

I originally thought it could be a corrupted DVD, but I have tried a newer map DVD (2008.2) and this has the same problem. I am thinking maybe it is the actual DVD that has an issue.

Anyone seen this before?