Started getting 404 errors on certain posts

Hi,

I noticed my visits were down a little this month so I logged into Webmaster Tools and was informed that my sites overall error rate is 28%!!

On closer inspection it seems that a large number of valid posts are now returning 404 errors but I can’t see why.

For example this works fine:
http://www.adobetutorialz.com/articles/30972365/1/create-a-photo-manipulation-of-a-lonely-child-in-photoshop

Yet this does not:
http://www.adobetutorialz.com/articles/2411/1/Shine-Animation—Shining-Bodyparts

Both posts are valid and I can locate the second post in WP admin without any problems but if I click on View Post I get 404.

I have tried resetting the permalinks array as suggested in various websites without any luck and I have also confirmed that the permalink used is correct by using Debug This and looking at the rewrite query.

Query matched:
articles/([0-9]+)/1/([^/]+)(/[0-9]+)?/?$ index.php?p=$matches[1]&name=$matches[2]&page=$matches[3]

I have also tried disabling all plugins one at a time without any luck.

Can anyone think of a reason why some posts might return 404 and others not?

Thanks