Hmm...I'm running my Citadel in a VirtualBox VM which has been allocated a single core. The underlying server is running a quad core CPU.
I find that if I remove the offending spool file, the issue goes away...I can always identify the offending spool file, easily, as it's the largest spool file in the spoolout directory...continuing to grow but never sent/picked up.
I haven't encountered it in quite a while, though.