U4919DW, horizontal lines watching YouTube video

Just received new U4919DW monitor. Changing resolution from 5120x1440 and setting YouTube playback to 720p is not resolving the horizontal line issue when trying to watch YouTube or other online video. I do not have this issue watching same videos through Dell 2414 monitor. Next steps? What is the underlying cause for this (e.g.,PC's video drivers)? Are there minimum video driver requirements for the Ultrasharp monitors?