... try and find out what it is that waking up every four hours and causing those regular spikes.
That could be fun... not 
Good luck.
A process of elimination led me to a possible answer. Almost everything with a wired connection to the LAN is powered off for several hours during the night (even my mail server) including around the time of one of the spikes so that eliminates them. Everything with a wifi connection is prevented from generating the spike because my WAP is powered off all night.
So that only leaves the router and a VoIP adapter.
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Andrue Cope
Brackley, UK