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.