Inconsistent HTTP Errors from Uploading Images with Uploadif

I'm having great difficulty in finding a bug preventing some uploads to
my rails app through Uploadify.

Here is the error I'm receiving:

http://j.imagehost.org/0353/uploadify-errors.png

The thing that's making this difficult is that it's not at all
consistent. Sometimes the images will upload perfectly fine. Other times
they won't. The same image(s) that fail on one attempt will succeed on
another. I really can't understand it, and I can't find any information
from the web about it. The problem persists across browsers, operating
systems and users. Sometimes it works, other times it doesn't. In any
given list of images to be uploaded in one go, it's possible that all
will work, all will fail, or some will work and the rest fail.

I've updated all the related software to latest releases (except Rails,
running 2.3.4 there). Paperclip and Uploadify are running latest
releases.

If anyone has any advice on where I need to start looking I'd really
appreciate it. If you need any other information from me, just let me
know what and I'll get it.

Thank you very much.