Strange fault on TV at Dee Why NSW

Hi all!

A problem to exercise the knowledgeable amongst you.  I have an LCD TV
(analogue) in the Dee Why NSW (Aust) area.  When any advert which uses
printed captions is just about to finish, the printed stuff jitters and
there is a loud 50 Hz (?) buzz.  This does not happen with any other
adverts.  Is it some artefact in the TV transmission chain, I wonder?

BTW, apologies for the anti-spam identity..  I once made the mistake of
using my real email address in a ng, and immediately got loads of spam
including some which masqueraded as Microsoft "security" patches, and
many purporting to be from eBay or Paypal (still getting them almost
daily - and even in french!) and telling me my account was frozen and
entering my password would restore it!  Anyone else have this problem?
Microsoft reponded to a fax about this (was bogus email). eBay have yet
to respond.

I giuess that's two questions.  Thanks in advance.