I’m experiencing strange thing with Dynamic text boxes and how text (font) is displayed.

Here is my problem:
I have two text boxes in my swf.
Text box 1 is a ‘Static Text’ text box which display text using one usual font (ex: impact)
Text box 2 is created with the function ‘createTextField’ and display text got from an external text file, using the same font as Text Box 1 (ex: impact)

When viewing the swf file with the Flash Player, the characters displayed by the Text Box 1 are smooth, and nice to read. But the characters displayed by the Text Box 2 are rough, irregular.

I have tried different things, but without success.
That drive me crazy, am I missing something ?

Do anyone know what to do ?