This has happened once to me. I was playing basketball, my foot slipped, and I felt my knee give, but I also felt my knee resist giving - which was probably the graft resisting.

The underlying problem - the muscles surrounding my joint we not strong enough to withstand the forces of my weight. It was back to the gym for me. Tons of work jumping and landing on one leg. One leg squats. Lateral hops. Foot speed drills (like on Hard Knocks). Core workouts (this was probably the biggest difference maker). I have been doing leg workouts since i was cleared to lift, but only in the last year have i been doing core workouts regularly and I have noticed the biggest gains. I have since been back playing basketball - full speed and it feels good. And I still ice my knee after every workout and run.