ssl vpn disconnects when heavy traffic flows through it

[fortinetuser2020]

fortigate 80c fw 5.4.4

we tested the problem from 3 diferent locations using the latest forticlient

when in ssl session, if you make a heavy action (copy file/watching video via rdp) the session disconnects within a few seconds

at the 80c side, nothing falls. the internet continues to work with no outage

any ideas?