Huge Traffic Drop. Lost keywords in SERPs

I run a spanish community - www.elforro.com - wich had around 200k visits per day in last October. In november the visits went down to 150k aprox.

But in december 17th we moved our servers to other datacenter and the site was offline a few hours, and with several problems a few days.. Since then, the traffic is about 100,000 visits per day and it's not getting better...

The 90% of the traffic comes from Google, so that's the problem. There are a lot of keywords that were in the first page of Google's Serps and now they disappeared...

Here's a screenshot of my stats



According to VBSeo Sitemap Generator GoogleBot is crawling about 80,000 pages a day... But the "indexed pages" shown with link:www.elforro.com keeps on 490,000 (someday it was over 1,000,000)

Do you know if there might be something else wrong or this might be just because of the problems with our servers since December 17th?

Thanks for reading!

Did you make any significant configuration changes when you made the move?

Any new software, upgrades, or configuration settings changes?

Your down time was limited to just a few hours, or for most of the duration of the few days?

Did you change your URL format, or keep it the same?