High CPU on one core, but unable to find process responsible

[David Rosenstrauch <darose@xxxxxxxxxx>]

My server's been exhibiting some very strange behavior lately.  Every 
couple of days I run into a situation where one core (core #0) on the 
quad core CPU starts continuously using around 34% of CPU, but I'm not 
able to see (using htop) any process that's responsible for using all 
that CPU.  Even when I tell htop to show me kernel threads too, I still 
am not able to see the offending process.  Every process remains under 
1% CPU usage (except for occasional, small, short-lived spikes up) yet 
the CPU usage on that core remains permanently hovering at around 34%.  
The problem goes away when I reboot, but then comes back with a day or 
so.

I'm rather stumped as to how to fix this.  The server is a bit old, 
running an up-to-date installation of Arch on a Intel Core 2 Quad Q6600 
CPU.  Any suggestions anyone might have as to either what might be going 
on here, or how to go about debugging it would be greatly appreciated.

Thanks!

DR