wordpress.com upgrade problem I have

Hi there
Since I updated wordpress 4.0, couple date later, my site have problem.
I use Hostgator of my hosting and after updated, my post link is setting( mydomain.postname) but recently if I click on it will turn to 404 failure page and show nothing relate to my blog. if I change Permalink to Default will be stable, but I want my Permalink always by post name.

This problem is bother me like a week, and sometime I change Permalink setting by post name is fine, but something will fail.

Please help me out with this!
Big thanks!