Video with the Nikon 810

[Garyk1108]

I primarily photograph and shoot video of horses. I recently received as a gift a new 810 and I am VERY pleased with it. Great camera! One problem I'm having is when I'm shooting video the image isn't always in focus, especially when the subject is running toward me at speed. I have tried setting the camera in aperture mode and also in program mode and neither seemed to solve the problem. Since video can only be shot in "live view" it's very difficult to tell if the subject is in focus or not. Is there a better camera setting that I should try? Does anyone have any suggestions?

Thanks for any help anyone can offer.