This sounds like a really strange problem. I am thinking a little out
of the box here, but this sounds like a classic clustering issue. One
server within the cluster has a different code set or is not working
correct. Hence you get this case of "it sometimes works and other
times it doesn't". Double check that all your mongrel, or whatever you
are running, have restarted properly and are running all the same
I would highly doubt that this is problem with your flex code if you
can upload the file properly, even if it is really small. Double check
that file is not corrupt on the other end though. If you could replace
your flex code with something else that can handle the same sort file
transfer, that could isolate the problem.
The only other thing I could think of is a bug within the front end
server which is handling the file transfer.