Are all serial cables created equal? I think we know that the answer is no, but we’ll give you a few specifics to ground this.
A Korg (not that Korg actually makes it) serial cable works fine between an Alesis QS-8 or QS-7 and a PC, but does not work reliably with a QS-6. This is confirmed by Alesis. A cable from Alesis works fine with all three keyboards. The Korg cable works fine with all of their keyboards. Is it the cable or the QS-6? Who knows?
Not too long ago one of our techs had a 15′ generic serial cable between his Mac and a MOTU MTP-AV MIDI interface in a large keyboard rig. It worked fine, except on sys-ex transfers, where it would sometimes freeze the computer or at least not complete the transfer. He removed the serial cable from the pile of other cables it was running with and the operation improved. He then replaced the generic cable with the cable that comes from MOTU with the MTP, which solved all problems. The generic cable worked fine in another system. Most likely this was a shielding issue, but who knows.
The point is, if you are having strange problems with MIDI data it is not necessarily a software problem or a problem with the device. You should definitely make sure all of your cables are operating correctly. The best way to do this is to try swapping them with new and/or different cables.