Hi,

I do not know about the PM9600, but IIRC it has a fast-SCSI (8Bit, 
10MHz) SE bus while the 29160 uses a LVD (up to 160MB/s ?) 
bus. An LVD subsystem needs special cablings (looks like standard 
UW 16bit cables, but the signal and ground (?) lines are twisted) 
and an active terminator (here again, a UW or other physically fitting 
terminator will not suffice). 

You could try to hunt down the problem (if you do not use LVD 
cabling/termination) by limiting the IBM drives to SE mode (jumper 
on the drive - I think it is the leftmost, but you will figure this out) and 
if possible the 29160 too (though I do not know if this can be done, 
the 2940UW for x86 allows to do this for every device). 

Some adptecs have problems with their autotermination function, 
set it manualy to see if it helps.

Some G3/G4 cards (for older PMs like the 7500 and probably the 
9600 ?) have been reported to cause problems with OF aware 
adaptec controllers under MacOS. Probably this could be related.

HTH

mike