.0 releases of FortiOS are notorious for having bugs. If this is a testing/lab firewall then go for it, but if you are using this in a production environment you are better off using a later 5.6.X or 6.0.X release for stability.
For example at work we are still on a later variant of 5.6.X for a 500D supporting hundreds of users, while at home I have a FWF60E and I'm running 6.0.3. I had some issues with DNS and 6.0.4 so I haven't gone past .3 yet.
We just grabbed a pair of 501s, and we will test 6.0.4 on those before we put into production with the assumption that the newer hardware on the 501Es will handle 6.0.X better.
I know it is tempting to get access to the latest gizmos, but on a FG of your size you are going to be constrained as to what you can do. It may help to review the features enabled and turn off anything you are not using such as WLAN or Switch Control, Adv Routing, etc.
If you do open a ticket, you can SSH into the gate and capture some of the following:
diag debug crashlog read
(If that is insanely long, use "diag debug crashlog clear", wait 24 hours and send the new output)
If the UI is crashing and you can login, you may need to keep an open terminal session up via serial to capture the output, or enable Syslog.
Here is an article with more info: https://help.fortinet.com/fos50hlp/54/Content/FortiOS/fortigate-toubleshooting-54/troubleshooting_tools.htm
for 5.4 but still useful.