Frequent Internet disconnections

Hi all,

I seek your support for resolving internet disconnection issue. In my new network scenario, Modem is in bridge mode and Fortigate 60D is configured in PPPOe. From this device connection is given to NW switch & a Wifi device. My network is a small workgroup without any DNS servers. All system Ip address are configured static with google public dns. 

Now issue is that, users are facing frequent internet disconnections (5-15 seconds) at least 1 time in an hour. Pls help me resolve this