Music on iPhone either skips or does not play at all?

I've had several different iPhones for 3 years now. During the 2nd year of my 3GS contract, my music on my phone began to skip randomly or not play at all. This wasn't on all songs, but happened to most. I then took my 3GS in and had it swapped, but the problem still continued! As I couldn't be bothered with the hassle, I chose to leave it as I would be upgrading within a few months. My contract ended in November 2012 and I then got an iPhone 5. As it was a brand new phone, I thought there wouldn't be an issue with the music, but it still continued! I figured if I went and swapped my phone again the problem would still persist so I have not bothered. All of the songs are bought from iTunes directly or either from an album off my computer. None are illegally downloaded. Sometimes when buying a song from iTunes, it will work for about a week then either start skipping or not playing at all! It seems that it isn't a problem with the device but the actual software I'm carrying. Any suggestions?

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