loses connection to the Internet

Are you using any FSSO ?

Hello ahmedhamdy,

When the issue happens, do you see traffic on Foritgate from that user?
Are you able to ping the Public IPs when the issue happens?
Sometimes it could be a DNS issue.
If you are using FSSO users then make sure the users are present in the Fortigate FSSO logons and matching the group. 

You can run the following sniffer for your source IP and then ping the Internet IP address to verify that the traffic is reaching Fortigate or not:

di sniffer pack any "host x.x.x.x and icmp" 4 0 l

x.x.x.x is your source PC IP address.

Try to ping the IP address and the websites you are trying to access.