Are you running any control panel, like cPanel? It could be a service like ClamAV or the common pkgacct process that is known to hang for long periods of time and cause higher server loads.
Is there a spiked server load when the site slows down?
Have you considered that it may be a problem with your host? In other words, you may be on a bad vlan (do you share an uplink?), or they may be rebooting their switches due to constant ddos attacks, etc.. and some of these switches have bad table routing. The IP that devnet is on... is it shared with any other websites? If so, it could be a DNS issue... your DNS could be incorrectly cached for various points in various DNS servers. This likely isn't the issue if you use custom nameservers and host your own DNS zone files... but if your host manages DNS for you... they could be mucking it up using zone files that are automatically regenerated every X minutes on their DNS server.
