Mongrel users - Sep 2006 - File uploads intermittent upon first form submission

I have Mongrel 0.3.13.4 running Rails 1.1.6 behind Apache 2.2.3 with  
mod_proxy_balancer, mongrel_cluster 0.2.0 as per the documentation,  
and mongrel_upload_progress 0.2, on Fedora Core 4. I''m using a  
cluster of three Mongrels.

I have an existing app which allows users to upload files, and I''m  
migrating it to Mongrel from Lighttpd. Uploading files via multipart  
forms fails intermittently, using both my existing code and the  
mongrel_upload_progress example files/upload action. (My existing  
code doesn''t reference anything to do with mongrel_upload_progress at  
all, so I don''t think that''s specifically at fault.)

When it fails there is apparently no attempt to upload the file - the  
packets aren''t being sent from my laptop and the form just sits  
there. A second submission of the form will work. Refresh the page,  
select the same file to upload, and maybe it will work first click,  
maybe not. Rinse and repeat. Small files less than 100kb seem to work  
first time, every time.

Can anyone suggest an avenue of enquiry to try and work out what the  
problem might be?

Cheers,

Mike