I think you are seeing the same effect as I see when trying to run an RS232 boot loader through a USB to serial converter. The boot loader seems to be very fussy about send/receive character delays and some converters, some of the time, work while others don't work ever.

I have even managed to blow up the bootloader through a USB to serial converter and had to reload the bootloader via EPIC.

I suggest you try several different USB to serial converters.

HTH
BrianT