Help with debugging - stall on response size is a symptom but what is the cause?

[LetsPlay]

Hi, I'm new to nginx and am trying to use it as a front end web server
to a java-based Play framework web app, via reverse proxy.  But I'm
getting strange timeout behaviour and am not sure how to proceed with
debugging.

The full details are at
http://groups.google.com/group/play-framework/browse_thread/thread/c853a579d6015d28/e6cf2a53333aa921?lnk=gst&q=nginx#e6cf2a53333aa921

I've been comparing debug-mode access logs for pass and fail cases, but
cannot fathom what nginx is doing.

How could I proceed with debugging?  I'm working on the assumption that
its my system configuration which is causing the problem - I haven't
seen any other reference to this kind of problem.  I'm running a box
with Ubuntu 11.10 x86_64, and nginx was installed from the Ubuntu repo. 
More than happy to provide logs.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,222240,222240#msg-222240