The lateral side of the leg above the knee would be under stress before the joint is put under load and this is the reason why it is popping in and out and the stiffness is the knee joint the next morning is cause by the irritation behind the knee cap and the joint. You can go to your doctor and ask "why" however in my experince they will not be able to provide you with the root cause behing your problem and will just treat the symptoms.
The knee joint like any other joint has muscle across that moves the joint and one of the biggest things that surprised me that this is not taken in to consideration. What you need to look at is to take the stress out of the muscle tissue in the hip, quad and hamstring and maybe the back of the calf. This is what’s causing your knee to give way when going down stairs because the muscle tissue is not strong because of the stress in the tissue.
You need to have the knowledge to take the stress out of that area, stretching and strengthening will not eliminate your problem, it may treat the symptom for a while but the joint will not be as it should be. Few people are aware that there are specific techniques that can be used in elevating these issues (the issue is in the tissue!) few specialist are aware of the importance circulation plays in all health issues and disease. The importance of circulation goes unnoticed throughout diagnosed symptoms when commonsense tells you that circulation plays a major part in staying healthy. Boosting the circulation is vital to all healing processes.
Clients are left to discover to try and work out on their own the solutions to their problems, this is why I have spent many years researching the “why” behind clients’ pain and frustrations as few professionals can provide you with the answers of “Why”. Go to any professional and continue to ask “why” until they feel uncomfortable during this process. This is why I have now developed my own techniques, that are not only bringing results that alleviate pain after many years of suffering but at the same time I teach clients’ how to empower themselves to take responsibility of their own wellbeing.