My 85 year old Dad is really in good shape for his age, maybe about 20lbs overweight, but what really concerns me is that about once a month or every so often he tells us his legs get watery, and they just collapse. He is fairly active for his age, doing some minimal yard work, volunteers at a shelter once a week, and is still able to drive well. Yet, while walking a block to the shelter, his legs just gave out and he fell. Also, while raking in the yard for about 1 hour, he also fell. He gets only cuts & bruises so he thinks this is no big deal. What could possibly be a cause of this???