movies with D7000
I suspect that the shift to dark, assuming it happens a number of times is due to the change in the exposure settings. Set your AE-L/AF-L button to AE lock (hold) and when you initially focus, press that button to lock in the exposure. Pressing it a second time releases it. Other than that use a tripod with a panning head and don't pan too fast if possible.
As far as possible, without surrender,
be on good terms with all persons.
-- Max Ehrmann