There are many factors the can contribute to flicker.

Here are things you can do to reduce flicker:

• Make sure your lighting is controlled. No natural light coming through windows.
• Make sure your power is consistent. Fluctuating power leads to fluctuating lights.
• Watch out for reflections. If you wear a bright shirt, it can reflect light onto the set.
• Use longer exposures. A one second exposure will be less likely to have flicker than a 1/100th exposure.
• Use the ‘capture blackout’ preference in Dragonframe to turn the screen black during hi-res capture.
• Use manual aperture lenses. The opening and closing of the aperture can lead to fluctuations in the exposure level. This is covered in another answer.