If you're looking for consistency I'd try and stick with manual settings, you definitely don't want to use auto ISO while shooting video. Your shutter speed is fixed, so that stays at 1/50. You want to keep ISO as low as possible of course, and set a limit for how high you're willing to go. On the 5D3 my limit is ISO1600 for professional work, maybe 3200 but it's never come to that.
If everything is maxed out then add more lighting. On a sunny day even at ISO100 you're going to have to stop down quite a bit, so I'd really only be concerned about low-light situations. Either way, I wouldn't try to shoot an entire short with no lighting, it can make all the difference in the world (regardless of how well these things do in low-light).