just an update, with some help from a member (thanks lynn) i think i found the problem.
when ad blocking is enabled in zonealarm, it was sensing an html comment(!) that had the word "AD" in it, and getting completely confused.
the html comment is used on our pages to help us remember when the google ads can get inserted - i've never head of a blocker looking at html comments.. interesting.
anyway, i've changed the html to remove that word and it seems all is fixed.