What causes weakness/stiffness in knees upon standing?

I am experiencing weakness in knees upon standing as well as an inability to straighten legs completely without a pause.  I feel no discomfort after a few moments.  I also feel pain when sitting with knees bent too quickly.  What causes this and are there any stretches or exercises I can do to correct this?  
(I also have occasional hip pain (left side) accompanied with numbness in both pinky toes and the ones beside them.)

[Adam Tanase, D.C.]

I would recommend you speak with a physical therapist about this issue.