Crazyaussie

I can tell you it is not because you have 2 monitors, I have always used 2 monitors for streaming and movies, and I have no worries with it, I have an i7-3930k CPU, I have not used an addon for any castbar yet, what is your Addon memory set to? That might have something to do with it, also try uninstalling your castbar addon and deleting all the settings for it in your WTF (You can either delete the whole WTF or just go through and delete that particular addons settings, I prefer the deleting of the addon settings because it removes saved settings for other working addons) Hope that helps, can always give it a try, to increase the Addon memory in your Character Select screen click on addons and increase the number in the top right corner, you can have it to 999MB, I am not at home at the moment so I am not sure whether you can increase it more via a config file or something in the game files. I will check and update this post when I get home. Good luck!

Velccie I would agree that it is most likely due to Latency issues, but that being said I would sometimes have that issue with castbar addons, but I have read you said that you tried without any mods, so I would chalk it up to Latency issues, can you tell me what your latency is at most of the time?

I am not exactly sure what you're having an issue with, if it is the way the casting bar reaches the end, it kinda looked a bit jagged I suppose, but the only way I can think of to make that look smoother is to change some settings. Here is a link to a guide to increase both Graphics and Smoothness. Hopefully that will help you.