After sitting and rising left leg threatens to collapse .once several steps. all ok some pain in calf?

Leg weakness: This scenario suggests extension related or change in posture related nerve compression. If persistent should be evaluated.

Possible knee derang: One must investigate the possiblity of an internal derangement in the knee. Pathology such as a meniscal tear(most common) or loose body might also cause such symtoms. Also evaluation for possible patella instability might be helpful.