2 yrs ago and b4 i was haven the same trouble with my knees, then 1 day i was laying on the couch watching TV. i got up to do something and my knee bluckled on me.
by that night i wasnt able to walk on my right leg at all. and the next morning i had to go to the ER. as i wasnt able to walk or place any weight on my right leg.
my right knee was 3times the size it should be. after a MRI witch showed i tore my ACL and Minniscuse ( not sure how to spell it)
i had to have sergurey on it. its been over a yr now and i am just now getting to walk halfway right now.
so if your knee is giving out on you, be very careful as it can lead to big trouble. use a brace or something to give the knee support. it may save you in the long run.
still looking for answers.
Remember that advice is free, its your choice what you do with it. :)