We have a small (around 40-50 PCs) network with a Windows 2008 R2 domain controller and a FortiGate 60C.
Recently we started having problems where websites load incompletely. The main HTML may load, but images or the CSS sometimes don't load. Hitting Refresh only loads different images but rarely the complete page.
Sometimes the issue seems to be fixed for a while before coming back. Everything was working perfectly, and then it started suddendly a few weeks ago, and it has been getting worse (more frequently).
I have tried connecting a laptop directly to the DSL modem (which usually has only one client, the FortiGate) and everything worked fine. Connecting the laptop into the FortiGate network again shows the problem.
This happens not for one computer, but for most if not all in the network.
We have checked the DNS configuration in the DC and the FortiGate and everything seems to be OK. We really have no clue on what else to check. Could it be a faulty FortiGate? A DC problem? Anything else?