I need some help here because I'm at my wits end.

I own a medium size site that has had excellent Google presence for the past 7 years. I have some very high-traffic pages that differ only in a URL parameter - but the contents of the pages differ entirely. No data is duplicated, no content is duplicated, or anything.

A month ago, I became aware that Google was consolidating all search results for mypage.php to mypage.php?id=4, which makes no sense as the most popular page is mypage.php?id=1

I added a sitemap with explicit references to the different urls, added the HTML canonical URL tag to each page, went to webmaster tools and resubmitted the sitemap and each link individually, and went to webmaster tools and specified that the id query string is NOT to be ignored and specifically defined its behavior as specifying and differentiating page content.

A week went by and a distinct search result for mypage.php?id=1 went back into the Google results and I thought it was all over... until today when I find that all mypage.php results are being consolidated as mypage.php?id=1

What can I do? Where did I go wrong?

Search terms: "EasyBCD" should go to neosmart.net/dl.php?id=1 and has been in 1st place position forever. Now it's no longer there, but somewhere around position 6 is the result pointing to neosmart.net/dl.php?id=4 - a completely unrelated page. :(