Doktor, the time is not right for me to change versions. Do you ask because you have seen this problem in pre-Ipswich 1.1, and now see that it doesn't occur in Ipswich 1.1? I assume so, as you're a such stickler for staying on-topic ;-)
Thanks bitman, but I don't think that's it in this case. It will work correctly, say, 35 times in a session and then just stop. Relaunching Sonar is the only workaround I've found. This doesn't bring my world to an end but does seem to be a bug. I'll report it to the bakers. Though still keen to know if anyone else has seen this.