Intermittent service.

Hello community. I have a 100d fortigate that implements two months ago in network of 70 users, I have no complex rules, 1 blocks navigation cell, 2 out to all teams, the problem that occurs to me is that approximately 15 days ago, the internet service began to be very intermittent, intermittent understand that you do as a ping from one station to go 20 ping 8.8.8.8 and lost 10. Diagnosis I have done this as follows. • Update OS him to version 5.4, before was 5.25, did not work out. • Verify that the problem is the internet service provider, the service is ok • Restarting the FortiGate and service works well for about 3 minutes. • I turn off the fortigate 1 to 2 days and the service is working again for 3 or 4 days. • Replacing the fortiga by a simple Gateway and the service works really well. • I do ping from the network to fortigate and he says no problem •I do ping from the network to the IP 8.8.8.8 but fall 10 - 20. • i do ping from fortigate to ip ping 8.8.8.8 and no breaks. Really I do not know what else to look, I was thinking I suddenly were doing some sort of denial of service attacks.