I purchased one, it was perfect. It's never an exact copy though because apple screens have squares which help coordination with games and writing, but they still work fine, and there are no obvious differences from the apple screen.
Could be, but maybe a drop that you are/are not aware of? I would recommend a simple visit to your nearest repair shop. You may have a restriction set that you are also not aware of, so have a look in all of the obvious places where this could be.
The reason could be because of an accident. Most likely, water/liquid spills. Do you remember anything like this happening before? It could also be because of dropping it, quite hard, even if there doesn't seem to be any obvious damage such as cracks, freezing, resetting, etc. A ribbon cable could have also come loose, and if so, you should visit an apple store as soon as you can, because this could be dangerous! This can mess up the colours a bit, making random shapes (such as lines) appear on your screen. I would recommend simply visiting your local apple store for a repair.
This is happening because of a faulty battery, or because of an accident. have there been any recent water/liquid spills that you are aware of? It could also be because of being exposed to a quick movement or a drop. Done that? Also, have you at any time had it out in hot weather or really cold weather? If none of these seem to ring a bell, it's probably the battery, as it could have been used and charged up too often. Thats why you should NEVER charge your device up unless it's going to be a full charge because this will wear out your iPod's battery! How long have you had this iPod?
First of all, try everything on your ipod. Camera working? Can it charge? It may be what charges your iPod, so do not touch it and if everything seems fine, leave it somewhere safe, no where near anything flammable!
Replacing the battery obviously comes with a few risks of losing data... but you may have too little storage, too many apps, etc? When you are at the home screen click the home button twice to get the small menu appearing at the bottom of apps that you have recently closed. Every app added uses minimum 5% battery if it is left there, which means your device will black out because of battery being used by the apps. To delete them from the menu, hold on any one of the apps in the small menu. They should vibrate and a small red cross should appear at the corner of each one. Delete them! DO NOT WORRY! DELETING THESE FROM THE SMALL MENU DOES NOT DELETE THEM FROM YOUR DEVICE! THE ARE STILL AT THE HOME SCREEN! Remember to clear this menu regularly! If this does not seem to work, try syncing to a mac laptop but if that has no effect either, contact your internet provider or if you use wifi get it repaired. This should be free if your device still carries a guarantee.