Working on a project where the html text of a field would appear to jump/ contract on rollover. It appears the issue occurs when a dynamic textfield has autoSize set to true. There is a simple fix by setting autoSize to false after the text has been set.This may play with the height of the textfield if it is dynamically sizing so to fix this I add in the following code...
var preHeight:Number = _textfield.height;
_textfield.autoSize = TextFieldAutoSize.NONE;
_textfield.height = preHeight;