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I just thought of something else, if I do a robots.txt disallow to this parameter, wouldn't it kill all the rankings from external links to pages that contain this parameter?

Corwin - that's awesome info - thanks for posting the results of your tests!

You're welcome.  I hope it all works out as you expect.

Thanks Owen, I hope you are right.

Thank you very much Matt. Now I get it

Thank you for the reply Megan, just what I was looking for.

And now I'm back to the top SERP results for my main keywords. For example "price drop alerts" now returns my site as the 4th results after being gone from the first 5 pages for about 10 days. No major changes to the site or links to it that can explain it. I can literally pinpoint the exact hour it happen, yesterday between 6 and 7pm NY. Suddenly my Google organic traffic multiplied by 50 or even 60. I'm obviously very happy to get decent traffic again, but knowing the past patterns, this will only last for few days followed by few more days of being gone again over and over. If anyone has a clue what can cause this, I'd be very happy to know about it.