Is this issue occurring on all channels that you are watching or on a specific channel?
Try removing the channel from the Roku home screen by navigating to the channel tile, pressing the * key on your remote and choosing 'Remove channel'. Then restart your device from Settings>System>System restart. Once your player starts up again, add the channel back once more.
Have you also tried switching out the HDMI cable used to connect your device as well as using another HDMI port on your TV?
Does this clear up the issue you are seeing? Please keep us posted.