We may have come up with a resolution. I believe the situation is a compilation of a few different items. If we have all scanning on for HTTP, FTP, POP & SMTP, all IDS anomalies selected & SNMP traps set, we continue to have the CPU spike. If we turn all scanning off, it is fine, or if we turn SNMP traps off it' s fine, but we can' t run both at the same time. Obviously scanning is more important, so we have had to turn off SNMP traps for now, and are not polling the device at all.
So far, we' ve had SNMP off for 3 days, and have not had a spike since.
We are going to leave it like this for a week to 10 days to see if the problem re-occurs. If it doesn' t, then Fortinet 3rd level support is going to put in a bug report to the developers for this issue. They believe it is too soon at this time to attribute explicitly that SNMP is the culprit, but I believe it is. If you look into other responses on this post, you will see that another customer also had this problem, (which is why we decided to turn it off) and he is on an older build than we are, so it sounds like no build will fix this (we are on 2.8 MR4), which is the newest.
Is anyone else out there with this same problem, using SNMP traps and polling their fortinet from a monitoring station? If so, can you try turning it off & see if your problem persists?
Also, could you post a response to let the rest of us know?