Could be a lot of things. I suggest you test JGroups standalone first (so we can exclude a JBoss problem).
Take 2.2.7 (or 2.2.8 from the CVS) and run the same test.
I also tested it in the 2.2.7. And the result is same.
I use a new hardisk to install a new system and it is stable now. But the issue was not soluted in the old hardisk :(.
Any method for the checking?
Thanks and best regards!