FWIW, my 620 is doing the same thing. Shutting down for no reason and not saving the file. It’s a $49 one with firmware 2.33 and I can’t find firmware updates available anywhere. I think my Marantz days are done.
I am dismayed and sorry to hear about the issues for the new PMD 706, but I wouldn't lump in the PMD 620 with a newly released deck. In fairness, the PMD 620 was designed by Marantz under different ownership as this division has been bought and sold a few times. I have a PMD620MKII, and it has performed flawlessly for dozens of recordings, not a single issue ever. It is a great deck as is my PMD 661.
Added: I see in comments that the PMD620 offered for 49.00 has the original apparently buggy firmware. So maybe it was appropriate to lump it in with a newly released deck.Hopefully for those that ordered one on eBay, upgrading will help as apparently this original model was released before it was ready for prime time (sounds familiar
). FWIW, they had it figured out by the time they released the PMD620MKII.