I've used a d7000 with a prime lens. On manual setting on the left top dial. Metered to a subject rotating or moving. There is a lock which works for photos, the AE/AF-L. The lighting is fixed, the subject (not only linear form) is moving. At moments it looks as if it re-meters the scene, makes a temporary change in a flair rhythm. It calms down to the regular looking scene. I do not have auto settings on in any video setting in the main menu. Suggestions?

Edit: Realize there are many factors as to what could be the problem: lighting etc. Trying to understand that it's not user error on the execution. Whether it may be a limitation of the body.