We're seeing this same issue. I'd love to see some kind of response to this...
I searched around some more, and while I'm still not sure exactly what's causing this problem (some people have indicated it may be a bug in JSJaC, but I didn't see any details of what that bug might be) I did find recommendations to set the following system properties with the following values:
xmpp.httpbind.client.requests.polling = 0
xmpp.httpbind.client.requests.wait = 10
I set them and that seems to have cleared up the problem for now. Your mileage may very.