Download Firmware fw_pl3507A_d042204(for Revision A chips) - Not actually any use unless you got some fancy firmware flashing hardware!
Download Firmware fw_pl3507B_d042204(for any other PL-3507 chips) - Original firmware update I had, has worked for a few people
Download Firmware fw_pl3507B_d0907(updated Prolific firmware I've got, only for B and C chips) - A newer version, worked for someone.
Download Firmware fw_pl3507B_d100604(updated Prolific firmware I've got, only for B and C chips) - Yet another newer version, but no feedback on it yet.
Download Firmware fw_pl3507B_d110904(updated Prolific firmware I've got, only for B and C chips) - Newest version, worked well for a user who sent it in..
Please download whatever you need. Read the included readme.txt in the zips to see the changelog and to figure out which RomWriter to use with the firmware. Please note that I myself have not used these firmwares because I have an A chip which supposedly can't be flashed by software alone, so I can't verify that these actually work. I offer no support for these because I can't get them to work :) So once again, USE AT YOUR OWN RISK.
If you do download the firmware/updater and it fixes your particular problem, please let me know. Of particular interest to me if you can successfully flash it is if the new firmware actually resolves the maximum 128k transfer in one IO. If you could download the testapp from BusTrace(http://www.bustrace.com/products/delayedwrite.htm) and run it after a successful flash and let me know the results, it would be much appreciated.