Also never seen that! Have you activated YT-related scripts in menue window/scripts? Maybe double/triple-buffering prefs have an influence.

But I recommand you to install YT-Url-extractor in OWB from Thomas Igracki (https://www.morphos-storage.net/index.php?page=Network%2FWeb&file=YouTube_URL_Extractor_2.1b.lha)
This will play YT-Videos external via MPLAYER. CPU-load is much lower and you can do anything else parallel. You also may use a different screen - just let MPLAYER-Window jump on that sreen to watch and MPLAYER-window can be zoomed ;).