left leg gives way, does not move normally, must move it voluntarily, cannot walk normally, and fall

left leg just gives way. very difficult to walk. must think of moving the leg otherwise it drags. Outside of left thigh is tender, but not expeeriencing pain, unless poked on the thigh. Have fallen several times. Parkinsons, myastenia gravis, NPH, PLS, Lyme disease all ruled out. Help! Please!