Sounds pretty wierd.  Have you tried putting the modem on the other serial
port to see if it's a Mac hardware problem?  Have you tried looking at the
port lines with an oscilloscope or voltmeter?  Have you tried turning the
Mac off for a while to see if the problem changes?

FWIW RS-232 says that you should be able to cross-wire any two lines in the
interface indefinitely without damage.  That doesn't mean you don't get
damage if there is broken hardware putting out high-voltage or something.

Good luck.

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