Cannot connect to the configuration database

Hi

I have a SharePoint 2010 server running on a VM and another VM running a AD + SQL server. The Sharepoint server got quite frequently cannot contact the SQL/AD server so it thrown out "Cannot connect to the configuration database" when client trying to connect to the SharePoint server.

After we restart the SharePoint server, everything will be fine. However, after two or three days, it happen again... 

Do you know why the SharePoint will behave like this... ?!

Thanks