Can you use the card to save/read data on your computer?
have you tried another card in the phone?
If so, could you use it ?
If you know the cards are good, then the problem must be with the phone.
My phone has also recently started rebooting itself, sometimes multiple times a day, then sometimes once a week.
This *could* indicate a serious problem with the phone.
If you haven't yet, try a factory reset. if that doesn't fix thing, try re-flashing with the stock ROM.
If, after all that, you still have problems, then the phone is most likely borked.