I have a website where Google Webmaster Tools reports 15,000 links as 404 errors. However, all links return a 200 when I visit them. The problem is, that eventhough I can visit these pages and return a 200, all those 15,000 pages won't index in Google. They aren't appearing in search results.

These are constant errors Google Webmaster Tools keeps reporting and I'm not sure what the problem is. We've thought of a DNS issue, but it shouldn't be a DNS issue, because if it were, no page would be indexed (I have 10,000 perfectly indexed).

Regarding URL parameters, my pages do not share a similarity in URL parameters that can make it obvious to me what could be causing the error.