Over the past 4 months, since redesigning our site, our main product pages have not been indexed. About 3 weeks ago, Google Webmaster Tools started to report that the pages returned a 404 (not found) status.
Going directly onto the pages or clicking through to them from links creates no problems and the pages load up fine with no 404 errors. We have looked at everything we can think of to try to work out why the pages are returning this error but have not been able to find anything.
What is the site? (You can type it in as text, i.e. example.com, without creating a link)
Have you checked what HTTP status code is being returned when these pages are accessed? Just because you can view the content does not mean the wrong status header wasn’t sent.
Have you checked that you can access these pages of your site in a fresh browser with cleared cookies without first visiting any other page of your site? This is how Google accesses it, so if visiting your home page to set some cookies is a prerequisite to accessing these pages, it won’t be able to.
A server sending the wrong HTTP response code is not a DNS issue. A 404 response means the domain was resolved, a connection to the server established and the HTTP request received and responded to. Far past the point of DNS in the chain of events.