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I doubt very much that an increase in old 404s resulted in a 30% organic traffic drop.  I'd look closely at your backlink profile, competition and page quality to try and diagnose why you saw that drop in traffic. As for the 404s I'd fix those that are fixable and 301 redirect the rest to relevant pages (or the home page).  If the number is extremely large then you should put a high priority on fixing this.  Otherwise I haven't met a site that Google couldn't find a 404 error on.  And yeah, they keep telling you about the same ones! Hope that helps! Jacob

I can't imagine how having multiple versions of a URL pointing to the same page could have any seo benefit. I would canonicalize the URLs. It shouldn't cause any problems with your tracking efforts either I'd think.