Why it fails to connect with github via SSH from TeamCity?

I have configured a VCS root with below settings:

- URL: the code repository from github.com

- Authentication method: Uploaded Key

- Uploaded Key: the key already imported to TeamCity

Then sometimes it is failed when clicking the Test connection button, and sometimes it is succeeded.

When it fails, I remoted the TeamCity server computer, and ping github.com with port 22, it could connect successfully.

I have encountered the problem for months -- sometimes it can connect, but sometimes it cannot by using the VCS configuration. The VCS checkout mode is as below

Any ideas on why it fails?