Since my original mail I've discovered that the problem is also occuring on another filer which is still running 5.2.2P1. I'm now wondering if it's more related to running Solaris 2.7 - the Sun system on which we now regularly see the problem was upgraded to 2.7 a few weeks ago.
My admin hosts, on which I see the problem, remain at 2.5.1 and even 2.5 (horrors).
Brian