Diagnose/Fix Internet Connection Drop outs (Zywall USG 20w)

I have a problem with my office internet connection. It seems to drop out every minute or two. I rang the ISP and they are saying that there is no fault on their end. I installed Total Network Monitor and I can see that pings to google are failing intermittently. When users are connected via Wifi the Internet Connection is stable. We are using a Zywall USG 20w, I need someone who has strong knowledge of this device. I have attached a very crude network diagram and a screenshot from the logs on the device.