You say that the sad card has plenty of space, but when was the last time you reformatted it in the bird using the go app? The directory structure is tricky and if you have been deleting files with the card in you computer for a while, you may not have properly reset some of the pointers in the directory. Files may show gone and used memory may show that you have plenty, but the phantom would look at the pointers and think that there was no room on the card and shut off the camera action. If nothing else obvious like a crash has happened, I would suspect the above is your problem. Try reformatting the card in the go app and see how things work. If that doesn't help, then you start looking for possible mechanical or FW problems.