SSL VPN crashing suddenly.

Guys, I'm having problems with SSL VPN. Usually in the afternoon my clients complain that SSL vpn access is out. When I'm going to run connectivity tests, I find that the registration of my account in the fortiguard service is not found. I can not even ping the fortinet DNS. I usually need to restart the ssl service, change and clear the DNS cache for the service to return to normal. In some cases, just restarting the fortigate. I use two FGT620 B in HA - v5.2.11. +/- 50 users vpn.