WAN Link goes down Every 10 to 15 minutes

This is Our Furewall detais
Model XGS87 

WAN Link goes down Every 10 to 15 minutes

It is installed on a dedicated Intel Hardware

Current Network Setup -
1. Airtel Fiber Link Router with Static IP lands on Fiber Link Router configured in Bridge mode
2. This modem is connected to the WAN port (Port 2) of the firewall through a Straight Through cable
3. And then the firewall’s LAN port (Port 1) is connected
4.
The Problem – WAN interface (Port 2) goes down precisely every 10 to 15 minutes.
Work Around - To resolve it, I simply go to
Configure Network Port2 and then without making any changes, I save the page and it works.
Troubleshooting done so far –
1. Replaced Fiber Link Router
2. Replaced Straight Through Cable between the Fiber Link Router and the Firewall WAN port (Port 2) with another Straight Through Cable
3. Tried Crossover cable as well between the Fiber Link Router and the Firewall WAN port (Port 2)
4. Tried Auto Negotiation and all combinations of Manual setting of Speed and Duplex at the Firewall’s end (Port 2)
5. Fiber Link Router does not have options to change Speed and Duplex settings.

Please prove me solution

I am looking at the experts in forum to help me resolve it. Thanks in advance