How do I make my Sony DVP NS530 play Region 1 DVDs smoothly?

I believe that my Sony DVP NS530 is region free. I've just bought some DVDs that are Region 1 and NTSC and when I play them, there is this flickering that appears on screen - the images don't jump, but the light flickers - it's as if someone is switching the lights on and off. How do I stop this from happening?

What settings should I be using on the DVD player, if I have to change my settings? I have also lost my manual, so a step by step process would be very helpful. Thank you in advance.