Using the instrumented kernel and taking a system profile will show if 
there's an external application that's interacting with io-net, causing it 
to run ready.  Another possibility that comes to mind is a bad driver 
which isn't releasing memory or is otherwise behaving incorrectly.  Make
sure that you have a shell that's running at a high priority (> 21) so that
you can still access the system if io-net consumes all CPU.  

For io-net to be consuming CPU, I would expect it to be either sending 
packets continuously or receiving packets continuously.  Have you 
checked to see what your network traffic looks like using some sort 
of network sniffer?

What type of network card are you using?  
Is there any information from sloginfo may be helpful?  
What does nicinfo report in terms of statistics?
What are the command line options that you pass to io-net?
Are you running io-net with any customized code (filters etc.)?
Does this happen on all three nodes or only one of them?
What does the memory usage on the system look like?  Does io-net's 
memory consumption increase with time (pidin -p io-net mem)?

(Lots of questions :->).

    Robert.