Welcome to the Nokia discussion boards!
Sorry to hear that you have been having issues with your memory. Are all your media and applications saved on your memory card or phone memory? Saving on the memory card can free up vital phone memory used to run applications.
Also, it may be worth your while to back up your phone regularly with Nokia suite, because this will prevent loss of data to minimal and also allow you to restore from that backup. But if you continue to have this issue, I suggest you to visit your nearest Nokia Care Point for further inspection of your device.
Let us know how you get on.