I don't think you have a mystery condition, everything you've said sounds like the normal aging process, older people generally have far more trouble going down stairs than up, eccentric strength being more demanding on the muscle and used far less in everyday life, is the first to go of course the more you avoid eccentric movements the faster it declines. You also need to consider your movement patterns when descending stairs etal . I was in a pub last night with a stairway, so took the opertunity, to carry out a study. What I noticed was that young d fit people walked down by landing on the balls of their feet, which then a) limits the amount they drop by say 4 or 5inches and b) uses the calf e muscle as a shock absorber for the knees The more dodery they were at descending, the more flat footed was there walking pattern, there is a chicken and egg thing here, as if there flat footedness was causing their issue or if there lack of confidence was causing them to be flat footed, but I suspect a main factor is the lack of articulation' of the foot and or weak calf muacles is causing them to land on their heel and over load the knees when taking the weight on their foot. Maybe bouncy shoes would help you !