Um, I forgot to mention that I am developing with 1.3 on RH Linux 6.2 with 256MB RAM, JBoss 2.2.2/Tomcat 3.2.2. I have tried killing all other processes during JBoss testing, but it doesn't seem to have any impact.
thanks for the help. Unsurprisingly it was my code at fault - switching to CMT gave me a more helpful stack trace. The constructor of the LoggingEvent object doesn't work in the MDB but does in a stand alone client.