Hi,
I believe you have specified a textFormat for the text. You need to add the following
// Assuming this is the format
var txtFormat:TextFormat = new TextFormat();
txtFormat.size = 20;
t1 = new TextField();
t1.defaultTextFormat = txtFormat;
// Apply the default text format that we just set to the text that's already in the text field ("g")
// Otherwise the default text format will only be applied to *new* text that's *typed* into the text field!
// Without this, the initial text in the text fields ("g") will use the default for a
// flash text field (Times New Roman 12pt), rather than the defaultTextFormat (Times New Roman 20pt).
t1.setTextFormat(t1.defaultTextFormat);
addChild(t1);
This sets all of the existing text in the text field to the specified format (making it the same as the default). Now when the text field is first rendered it uses the default format that you set, and when you type it also uses that same format, and so the format no longer appears to change.
Hope this helps.
Sanika