Excessive CPU usage over time

[Spaced Cowboy]

So I started using psensor yesterday, to monitor the new linux server in the garage and see how well it was coping with the ambient temperature. Here's the plot:

The spikes are fine - they're when smbd kicks in as the Mac does its Time-machine backup - verified by turning off TM for a while towards the start of the plot. What was a bit more concerning was that one of the dies on the 2990WX started basically staying warm about halfway through the test (the "blue mess"). The graph-window width is 2 days btw.

So I overlaid CPU-usage and got something of a surprise:

Something is ramping the CPU! Now the "blue mess" makes sense. 

This machine is literally idle - it's just been set up, there's no users logged in, and it's sitting there showing me a CPU ramp. Logging in and running top, I see:

Mystery solved, I think. Is there a way to prevent psensor from affecting the CPU so much over time ? Because if not, I'm not going to be able to use it :(