When using the Nikon D90 video mode you can sometimes observe a stuttering effect as it tries to adjust settings to changing light conditions. This can result in less than adequate clips, as shown below. Observe the stepping up and down of the lighting in the clip.
The video above shows the D90 adjusting to light conditions during filming (AE-L off)
This video shows the Nikon D90 holding it’s settings even with changing conditions during filming (AE-L on)
To prevent the camera hunting you can press the AE-L (hold) button before switching to LiveView (LV) mode.
Use these steps to set your camera for AE-L (hold): Menu > Custom Settings (Pencil Icon) > f (controls) >> Assign AE-L/AF-L Button > AE lock (hold) > OK