Another poster has suggested a 9500 logic board as a replacement. Having recently tried this, blindly following a thread in these fora, I can report that transplanting a 9500 logic board into a 9600 in fact will not work.
9500 logic boards will indeed physically fit (more or less) in an 8600/ 9600 case. However, the power supply connections are different, so that it is impossible to supply power to the board. Furthermore, a 9500 psu is physically very different than the psu in a 9600, and will not go in the 9600 case. So a functional 9500 logic board is no real use to someone wanting to fix a 9600, unless you are into serious hacking.
There is also the "Kansas" factor to remember: the 604ev originally in the late 8600s and 9600s used a different logic board than the earlier 8600s and 9600s, which were very like the 9500's architecture.
The failure of that well-used 9600 over time sounds to me rather like what happened to my 840av some 5-6 years ago when the capacitors first went bad. One simple diagnostic, after the pram purge, would be to give the logic board a careful wash and dry. If it then revives, you need new caps. It is a machine from 1996-97, after all. Its time has come.
