Just took the latest update 12.5.0 build 4182 for my Roku 3 and it is now glitching like crazy, can't watch anything on it because of the constant glitching. Never happened before and there is nothing wrong with the TV or HDMI cable as they've all been checked. This is a software update bug.
Have you tried restarting your Roku?
Depending on your Roku model you'll find the "System restart" under "Settings > System > Power", or if you have no Power option, under "Settings > System".