I can completely sympathize with you. As I was searching the internet looking up chronic knee pain I came upon your post. I am 38 years old and have had the exact pain you describe since I was 19 years old. I have seen 9 different doctors ranging from family doctors, bone & joint docs, sports docs and more. Every single doctor said they could find nothing wrong. My knees ache all the time though sometimes the pain is excruciating. When it is at its worst my knees will almost feel as if they "lock up" while they are bent, then when I extend them they will pop and crack and there will be a few moments of relief until the locking up pain comes back at which point I have to bend them again. This pattern repeats itself over and over. The worst pain usually lasts anywhere from a few days to over a week and then it will transfer to a lesser, though still chronic pain. No matter the severity my knees always crack and pop when I bend them. My knees also tend to give out and 8 months ago this happened and I fell down 12 stairs in my home cracking my tailbone. It has gotten to the point where I feel like I am just going to have to live with it. I have been prescribed different medications but nothing has worked. My mom believes that it is fibromyalgia, which causes chronic pain and fatigue, and there seems to be no real cause for it. There are many websites on the subject so you might want to check that out. I do have an appointment with a new doctor the day after tomorrow and I am hoping I can finally get a real diagnosis, though I am not overly optimistic because all x-rays thus far have turned up no problems. I will let you know if my new doctor can come up with a cause. My advice would be to perhaps seek out a new doctor and don't give up. If it is fibromyalgia there are many new medicines out there that seem to help others that are suffering from the same thing. Good luck to you!