First was that at some point corruption had crept into the card in the hidden files and folders that android created.
My fix was to Mount the card in my laptop and copy everything off which took some time because it's a 64gig card. I then formatted it in my laptop and reinserted it into the phone so it could be "prepared" then I put it back into my laptop and copied all the stuff back on then deleted the media database on android and them rescanned media to rebuild it.
No problems since.
Not saying this will fix your issue but it did mine