To answer your questions:
1. FlashFXP hangs when about 60% of the way through a file.
2. The UI at this point stops responding.
3. The transfer continues to upload in the background and eventually finishes.
4. It doesn't always hang at the same point, its a little random depending on the size of the file, but I can be sure it will happen.
5. MTU Settings in FFXP = 8/32
6. MTU Settings in DSL Modem = 1492
7. MTU Settings in WindowsXP = (default)
I have also checked when this last happened by doing a quick 10mb file upload and noticed that the PID for FlashFXP shows the CPU utilization at almost 100% (this I believe is why the UI becomes slow to update). It fluctuates from about 90-100% when this behaviour is occuring. Once the transfer finishes it returns to normal. Another interesting thing is the graph has a huge spike in it much more than I can upload at which is only 25k/s.
I will try to uninstall Zonealarm to see if this problem goes away and report it back but I am reluctant.
Look forward to hearing from someone if they have some more ideas or want a debug etc.
CPU
http://ruffin.customer.netspace.net.au/tmp/040.png
Graph Spike
http://ruffin.customer.netspace.net.au/tmp/041.png