Well, maybe it's a problem with the internal storage? I'd connect my phone to your computer, copy all of the files over, then format the storage (menu->settings->storage->format internal storage). I wouldn't copy anything back, just in case it was a corrupted file that existed before. See if the problem recurs after that? If so, then it probably wasn't it, and you can copy those backup files back. (In that case, maybe a factory data reset and reinstall apps from scratch to see if that solves the problem - but that is, obviously, extreme..)