Crashes indicate either a software bug (zero pointer reference) or a 100% memory consumption. This can be caused by a faulty process, a memory leak etc.
I' d try to switch off the mentioned malware features to see if it influences the interval between crashes. If you find anything that can be replicated, open a support call with Fortinet. They' ll fix it but only if it is reasonably easy and reliably to reproduce.
Years ago my 50A started to reboot all of a sudden, and only stopped when I disconnected the WAN line. Some evil network had hammered away at the poor box so violently that it couldn' t keep up with the traffic and crashed onto the wall. But from your description I don' t think you have a similar case here.
Ede Kernel panic: Aiee, killing interrupt handler!