Google not indexed since site map? consider this

Hi,
having had our site ranking and number of pages indexed totally
destroyed at the same time we introduced a sitemap, coincidence I'm
told, Jagger I'm told, poor pagerank I'm told, no inbound links I'm
told well bollocks your told.

Consider this:

A google sitemap contains a hint (supposed) as to how long a page is to
be shown, now if say that is for a month, as with a lot of classified
ads they are only around for that length of time or not much more, does
google look at that as not worth indexing? and would that be the reason
our site has been totally destroyed since introducing the sitemap?

Anyone got a good reason why this is not the case? and if it is the
case, does it pay to have an honest site and now should we be trying
to find ways to get around and mess up google.