Did you/can you set border="0" to dispense with it entirely?
ust one line is size 24 (correct) when the page is loading, and then quickly drops to size 12 (incorrect) when the page is fully loaded.
If not, it almost sounds like a specificity problem. Let's say you have
Which means for the class somestyle inside the object ID'ed as header, apply a rule. Then later in the css you might have
for some different element. Normally the .somestyle inside #header would be more specific and stick, but the second rule might be overwriting the first. It could be something LIKE that anyway, start removing CSS on trial and error.
Or, because of the delay, you might have some ad or something being included in the page affecting the applied styles.