Going down stresses knees much more than going up. Try slowing down on the descent and get trekking poles. Properly using trekking poles really saves your knees. Because going down seems easy (not gasping for breath) you tend to go too fast and not take enough rests.

I just came down 6,500 feet over 6 miles yesterday, in 6 hours. I kept a purposely slower pace than I felt I "could" go, took substantial rests every hour, and really used my trekking poles. My knees did fine. Whereas, last year I did a 5,000 foot descent over 13 miles, literally running at some points, and my knees killed me for weeks after.