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Sorry for the obvious questions but...
* are you using the same Keyspan adapter with both cables? (I think you are?)
* how long is your newly wired cable?
* have you checked the resistance/impedance for both cable A and B?
* have you checked inside the connector itself of the working cable for cross connections?
From what you describe, my first Occam's Razor thought would be that the Cable B is damaged, and the new cable isn't wired correctly... I've personally experienced Cat5 cables that superficially appeared fine but would cause intermittent corruption - Rs232 is usually more resilient though.
To do further diagnostics I'd suggest:
* run a serial port logger/debugger on your computer to see if the bytes being sent/received are different with different cables
* hook up an oscilloscope on the RX and TX pins to see whether there's an electrical difference between the cables, e.g. too much impedance or cross talk may give you poorly shaped square waves
Well, that's all I can think of right now. What a puzzler!
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