Hi SLF
Just a thoght but who is your ISP? My experience suggests they can remotely adjust your settings.
I use Demon but maintain a backup account with Virgin. My default connection is one or another of the Demon numbers. However on the odd occasion I connect via Virgin it is immediately reset to their dial up number. Never actualy investigated whether it also messes with the settings you mention but it would make sense from an ISP point of view as it makes it easier to force advertising down your throat!!