knee pain

I've had nothing but problems with my right knee since 2001 and am wondering if anyone can give me some advice. I had arthroscopic surgery in 2001 to repair some torn cartiledge and scar tissue that had accumulated in my knee. Then, in 2002, my knee just gave out and dislocated when I was picking up around the house. Now, I get this pressure build if I leave my knee bent for too long. It cracks when I straighten it and the pain goes away almost instantly. Also, my knee pops a lot and gets very all around but mostly under my knee cap. If I place pressure on it, the pain seems to subside a little. If anyone has any advice on this, please let me know. I had an MRI done about a year ago and went through 3 weeks of intense physical therapy which seemed to help at first, but now the problem is back and seems to be getting worse.