If you are just using the JMS component and you dont need clustering then I can reccommend SUN IMQ Message Queueing server. Its free in non-clustered mode, stable and very reliable, its not the fastest, but it doesnt suffer from the kind of performance degradation you are talking about, the admin console is sweet,...
Dont get me wrong, I love JBoss and we use it extensively, just not the JMS part. Your integration tier and services always need to be rock-solid.
My mistake, you are doing multi-consumer queue type work, thats not an available queue delivery policy in the free version of Suns IMQ.