Memory Usage goes high

Hi!

I am using Fortigate 100D with spec:

HA StatusStandalone [link=https://192.168.0.1/system/ha/monitor][Configure][/link]Host NameFG100D3G16829885 [link=https://192.168.0.1/ng/system/hostname][Change][/link]Serial NumberFG100D3G16829885Operation ModeNATInspection ModeProxy-based [link=https://192.168.0.1/p/system/vdom/edit/root/][Change][/link]System TimeMon May 8 18:50:57 2017 (FortiGuard) [link=https://192.168.0.1/ng/system/time][Change][/link]Firmware Versionv5.4.3,build1111 (GA) [link=https://192.168.0.1/ng/system/firmware/?viewOnly][Update][/link]System Configuration[link=https://192.168.0.1/ng/system/config/backup][Backup][/link]  [link=https://192.168.0.1/ng/system/config/restore/][Restore][/link]  [link=https://192.168.0.1/ng/system/config/revisions][Revisions][/link]Current Administratoradmin [link=https://192.168.0.1/system/status/status][Change Password][/link] /2 in Total [link=https://192.168.0.1/ng/system/admin/current][Details][/link]Uptime2 day(s) 1 hour(s) 42 min(s)Virtual DomainDisabled [link=https://192.168.0.1/system/status/status][Enable][/link]

In 2 day work, device will upto 80% memory usage, need reboot, if not reboot, memory will upto 85% then device will can not work, internet wrong, I did turn off and turn on device by button on device.