Inherited a client's site that was setup by some other IT firm using parallels plesk. Occasionally the plesk service seems to hand and stop serving http requests. Simple restarting the psa service fixes the issue. As we didn't set this up I'm not really certain where to start looking to debug. It's not got any dmesg or kern.logs and the bash options are highly limited.

It doesn't look like the server gets tapped out on resources since I setup some monitoring, and even when the websites stop working the server is still only sitting at 50% utilization. Before I sink a few hours digging around documentation, anyone familiar with Plesk and what might be causing it/where to look?

The version of Plesk is Parallels Plesk Panel v11.0.9_build110120608.16 os_CentOS 6. It apparently hasn't been upgraded for over two years.