My iPhone turns off and my battery percentage is stuck?

Around a month ago, my iPhone started shutting itself down, but instead of doing it straight away, the screen would fade in vertical stripes, and then it would turn back on. If my phone is charging, it will also do this but get to around 40% - 80% battery percentage and the battery percentage doesn't change at all. I tried to reset my phone but still nothing worked, my phone doesn't stay on long enough to restore any of my backups. Literally 2 days afters my warranty ran out, this began to happen. Why is this happening and how can I solve it? Thanks!

Added (1). I have an iPhone 4S, it's only a year old!

It might be a hardware problem with your battery. There's a software called iPhone Configruation Utility from Apple where you can see error messages. Install that, connect your iphone to computer, select device from list on the left and then select console tab on the right. It is ok to have a few error or warning messages but if there are many of them and they are periodical most probably it is an hardware error. Otherwise it could be a software issue and you could try to restore firmware in DFU mode.