First question to ask - are the other domains getting better rankings? If what you described is in fact what is happening (Google "discovered" the other duplicate websites) then one of these domains should be receiving the previously favourable rankings.
My experience is that when Google discovers multiple domains with the exact (or near exact) same content they will (almost arbitrarily) pick one of the domains and shunt all results to that domain. They won't punish you, though.
For example, a client of ours had 3 domain names. You would do a search for "keyword one" and "domain1.com" would appear. When you clicked on the link Google would actually take you to "domain2.com". Ditto for "domain3.com" and, of course, "domain2.com". Make sense?
This makes sense to me. It appears to be Google preventing people from having multiple domains (with duplicated content) gobbling up extra search engine results.
Setting up a 301 redirect (permanent redirect) is definitely the way to go. This can be done with the help of your hosting company. DO NOT use a javascript or an ASP/PHP redirect or you will be punished. Trust me - I know this from experience.
Good luck.
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