You see peroneal tendinitis come up often as questions in forums, so a while back I wrote a lengthy post on it that a lot of people refer to, which is great as I really wanted to get the information out on this.
It is a condition that I often see from comments in forums that is frequently mismanaged. I do spend a lot of time on it at my Clinical Biomechanics Boot Camp course and like to tell the story about how I used to hate seeing it as my success rate used to be close to 100% failure; now its close to 100% success. The change was that dramatic once i realized what was going on with it; hence my reason for writing the blog post that I referred to above.
HINT: it almost always occurs in those with low supination resistance.