Best firmware for Fortiwifi 90D

Hello All

I have the firewall mentioned above. The device has an active service contract. My device has crashed about once every two or three weeks for the past 6 months. I am on firmware 5.2.7. I have been on the phone with fortinet tech support and have followed their recommendations, but the problem persists. I have disabled local disk logging and configured it for syslog. Netscan was throttling my cpu to 99% leading up to the crash. The crash log lists scanunit as the last active process before the crash. I am looking at options, but I would like someone with experience to tell me what they think the most stable firmware is. I would very much appreciate it if someone can.

thanks