[c-nsp] Weird problem on Cat6509 + WS-X6348-RJ-45

[Jeff Kell]

I have now been through the *third* occurrance of a very strange communications hangup involving a WS-X6348 blade  and, at various stages, a PIX 515E and a 2611.  It is one of those non-reproduceable things that is hard to troubleshoot (at this point TAC would probably shrug this one off).  Any clues appreciated.  I'll try to make a long story short...

Communication stops on a point-to-point (/30) link with the neighboring device (100TX link on both ends of a short jumper cable).  No ping response from either end.  Tried changing cables.  Tried changing ports.  Tried clearing ARP.  Tried clearing mac-address tables.  Tried clearing interfaces.  No dice.  The other device can not communicate with the 6509.  If relevant, IOS is 12.1(26)E1.

Other interfaces, routing protocols, operation are unaffected.

6500 sees the neighbor and has it in ARP table.
Other end does not see the 6509's MAC (ARP entry 'incomplete').
Tried static MAC in the other end to no avail.

Reloading the 6509 'often' clears it, power-cycling always does.
Reloading the other device almost never clears it.
Power-cycling the other device sometimes clears it.

I can't reliably reproduce it, and since this is production hardware, I can't spend much time troubleshooting it (unless the 'other' device is not critical, as was the case with the 2611 recently).

Ring any bells?  

Jeff