Personally, I never noticed any real continuity issues, except for the occassional loss of a Rodan or a Mothra. If you go by what the films say, it's easy to imagine the Showa series takes place in chronological order, with the exception of DAM as the final film, being set in 1999. I hear a lot of people complain because Monster Island exists in the 70's films, however, this is irrelevant because Monster Island and Monsterland are two different locations. In fact, Monster Island was introduced first in Son of Godzilla, although this is only explicit in the Japanese version (both in the film's title and spoken dialogue). It's not like the people writing the films were unaware of the films made before, nor were they deliberately resetting the timeline like in the Millennium series.