I have a 5800 withe the latest firmware and don't have this problem, although I didn't have it with the earlier firmware either. This might sound stupid, but are you sure you're not mistaking the turn off as the phone locking itself? The slider button on the side will sort that problem out. Alternatively, I would suggest just reloading the songs onto the phone. The memory card may have become corrupt from a bad file transfer and phones can do some pretty weird things when they encounter bad files - it may be that it's turning itself off to protect itself from messing up. If it's on the exact same songs each time, then it is most likely a transfer error - it won't be a problem with the phone itself.