Sluggish Web UI - only reboot helps

[skywalker007]

I had a couple of situations recently where my firewall got very unresponsive on some services incl the Web UI. Logging into the Web UI then takes up to a minute.
The only thing that helped getting back to normal was a reboot then. How can I diagnose what'S going on?
Some logs on the CLI which I could monitor when it happens?
The system is a an Atom CPU C3558 @ 2.20GHz (4 cores, 4 threads) with 64Gb RAM and ZFS mirrored boot device, latest version installed, all updates.
It does run Zenarmor an I have seen mongod consuming quite some CPU cycles but normally that isn't an issue.
Any hint on how to track this down next time it happens is appreciated.
TIA!

[cookiemonster]

leave ssh enabled and ssh into it when it happens. Use top or htop (needs installing) when it happens and together with ps, it has a chance to see what might be consuming most resources and start narrowing down the problem.