Web browsing problem

Hello,

I'm having a problem with FortiGate 100E firewall.

We have in our network ( DC, ADC, Exchange, File Server, App Server and else.. )

we configured FortiGate and assigin the DNS settings in FortiGate to our IPS public DNS (NOT OUR INTERNAL DNS).

The problem is -- some time we cannot access to internet through google chrome browser) and when chrome work fine and access to internet getting internet problem through Firefox browser too..) i'm so confused.

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Network topology:

We have 3 internet line

All internet line connected to -----> Peplink balance (Private IP) -----> connected to FortiGate in WAN1 Port (Private IP) ----> connected to LAN (Private IP Diff subnet).

Any HELP.??

Thank you.