The issue might be the serial port. USB-serial timing has always been flakey with these things, and never completely sorted out.

I use a machine with a real ISA serial port when necessary, and the AirCable bluetooth-to-serial dongles for most stuff.

USB-serial *used* to work with download.c on the PC-side on Linux, but it got broken at somepoint, in the 2.6.2x series I think. I can boot a 2.6.16 kernel and do serial download over USB, but not with a modern kernel. Never had time to go hunting there, though.

Cheers