The OP was connecting a PowerBook (8 pin mini DIN) to PC (9 pin D). That sort of cable is hard to find nowadays. Ditto, pin over adapters. (Also a reason why some of us have cabinets full of old cables.)

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Trust me, I know why we all have cabinets full of old cables.

Tucson isn't a very big city, but I can definitely get db9 and db25 null modem adapters at the one and only electronic parts store in town. It might be worth a look.

Also, I don't really think the presence (or lack) of shielding should be impacting the OP in this situation unless he's in some very harsh rf environment. I agree that ZMODEM is worth trying.