This wasn't a problem with my stock FC2 install. After I ran 'yum update' it was still working good, until I rebooted. My guess is there is a problem in the new kernel (2.6.7-1.494.2.2).
Editing /etc/grub.conf to boot to the old (2.6.5-1.358) kernel solves this problem.
300KB/s no problem now, transferring the FC2 ISOs without a hiccup. Previously, even if I limited it to 100KB/s, it would stall out about 200-400KB into the transfer and basically hang.
Note that the problem is only uploading (transferring to) the FC2 host. I can download (transfer from) the FC2 host without any problem with the newer kernel.