Knee Pain

I'm still trying to find a publisher for my book 'The complete history of my knee troubles'. Admittedly the first 300 pages are a bit slow but it picks up after that.
I've lost count of the number of specialists and physios I have been treated by. I remember one consultant telling me I had the thickest patella tendon he'd ever seen which from a man who has spent his life looking at knees seemed notable.
I had what sounds like the same problem as you; I say sounds like because I don't want to give you false hope; knees, like spines, are complex. I went thru years of different diagnoses and treatments before a physio got it right; my knee cap doesn't move up and down as it should, it moves to the side. The solution was to apply tape to it to move it to the left and tilt it up a bit. I had to do this every time I rode (which was every day cos I'm a courier). This reduced the times my knee would 'go' from every few weeks to every few months. Though the condition was innate the problems only really started after I got hit by a motor bike so it was a combination of an existing problem exacerbated by a trauma.
Other things which helped; using a low gear (39x17 in my case), keeping the knees warm and not getting hit by any more motor bikes.
That was a few years back; now the problem seems to have gone away though I did have one episode recently after a spell off the bike.
So it's good and bad news; you can find a correct diagnosis and treatment but it may take a while. Equally a bit of RICE, as suggested above, may do the trick. Good luck.