phranque - 9:02 am on Mar 11, 2013 (gmt 0)
GWT = Google Webmaster Tools
AWStats is based on server access log files.
any best guess should be based on the best information available.
what is GWT reporting?
have you tried "fetch as googlebot" in GWT?
what kinds of errors and redirects are being reported by AWStats?
what is adsense telling you about which pages are causing the drop in income?
are you getting a drop in traffic to those pages?
is the CTR decreasing?
perhaps the clicks are paying less...