Maybe it is something to do with the following Cisco Field Notice:
Message Type : Field Notice
Title: Cisco Field Notice: Cat6xxx Switches And 76xx Series Routers Running A Sup2 With CatOS Versions 7.6(1)
Through 7.6(4) May Hang After Running For A Period Of Time
URL:
http://www.cisco.com/en/US/customer/products/hw/switches/ps700/products_field_notice09186a008026758e.shtml
Posted: July 27, 2004
Summary: Cat6xxx Switches and 76xx Series Routers running a Sup2 with CatOS versions 7.6(1) through 7.6(4)
may hang after running for a period of time.
We had 6509 which failed, because backplain failed (it can not happen -
but it happen) - iof course, no any 'dual CPU dual power' could prevent
it... Image broken line card - it can crash whole box no matter how much
'dual' things you have. The same with software error (I crashed one of 6509
just running 'snmpwalk' on it).
So, I always prefer to have 2 boxes and application level reliaility instead
of playing with 'dual everything' solutions (last example - 2 days ago one
of our dual-power Intel servers failed because of 1 power supply failure -
it did not broke, but it did something wrong''' and system crashed...).
Fortunately, 6509 reliability is very high and you must found someone wioth
thousands of 65xx to answer your question...