I believe I may have located the source of the issue as well as a workaround as reported by me in the bug report forums here: http://us.battle.net/d3/en/forum/topic/6934185046#1
in short, try disabling vsync if this is happening to you. if it fixes the issue, please reply here &/or in the bug report thread.
some users report once fixed, then can enable Vsync and it works fine, others have the issue return as soon as vsync is re-enabled.
if it doesn't fix, please reply here &/or in the bug report thread
hope this helps, thanks for your time & GL/HF!
this may or may not also be related to &/or fix the issue with fast scroll in locations other than chat, if so please report! thanks!
UPDATE: thanks to randallross for another workaround: try using your page up & page down buttons in chat, he says it works wonders!