@shapop62714 without actually hearing/seeing it and only going by your description, this sounds like a bad HDD. Pretty common for the year and model. At this point I would replace the HDD.
You replace it tthe same way as you would on the 6th gen [guide|564]
Yes, you will get a few answers that will suggest you convert your iPod to a SD card etc. but I just prefer to stay original. That way I do not run into compatibility issues. No, I do not need a device that will hold 15,000 songs :-)