What Causes Trouble In Walking In The Evenings?

Hi, I need to know that someone is having trouble walking in the evenings, specially after eating his meals. He feels like he will fall, so he does not walk or walks with very small steps and for very short while. Morning walks he is fine but this happens only in the evenings. It started happening few years back but off and on. But since a month now he is constantly having this problem. He had his orthopedic, magnesium, ECG, tests, spine test. everything came fine. What could be the cause of this.