When I plug in my headphones, many problems appear?

When I plug my headphones into my iPhone, a bunch of problems start happening.

1. Music and videos start and stop at random times

2. Music and videos fast forward at random times

???

I forgot what model my phone is but it says s on the back. I've dropped the phone a few times and the headphones are a bit old, one side doesn't work unless I hold the headphones in a certain position or press a certain area of the phone. My iOS is 9.2.1.