Created attachment 60595 [details]
Testcase for bug
Summary says everything. Sometimes it's "choppy" and sometimes it's just a jump before starting and ending position.
Works fine for me in Safari 5 on Mac OS X 10.6. There is some tearing on Leopard, which I don't see on SL - might be an OS level change, or just a faster machine.
Please provide more information about your configuration (platforms field says Linux, but there several completely different ports of WebKit that all work on Linux).
I guess you have another local than en-* or C ???
*** This bug has been marked as a duplicate of bug 18994 ***