I had the same problem on 2 systems, my MBP and also my iMac, both are Intel machines. The MBP had a new Mbox 2 Mini and the iMac had an original Mbox. FCP crashed frequently until I unplugged the Mbox on both machines. Due to deadlines I too just unplugged the Mbox and used a mini-jack to 1/4 cable for my monitors or used headphones. This is a recent occurrence, I’ve used this same setup for years with no problems, must be a something to do with a OS X 10.5.6 update. because there has been no updates to the Core Audio Driver. MY MBP has Pro Tools LE 8 and the iMac only has the Core Audio Driver.
Spent allot of time using Disk Warrior, Techtool Pro and repairing disk permissions, to no avail, so I wouldn’t waste your time on that.
If you can live without your Digidesign Hardware for awhile I would wait for another update to fix the problem.
Jesse
PowerPC G5, 4 x 2.5 GHZ, 16 GB Ram, Kona 3, Dual Cinema 23′.
Pro Tools HD 1, 192 I/O, D-Command Main, Neumann TLM 193, PowerPC G5, 4 x 2.5 GHZ, 6 GB Ram.
17′ Macbook Pro, 2.6 GHz, Intel Core 2 Duo, 6GB Ram.
24′ iMac, 3.06 GHz, 4 GB Ram.
PowerPC G5, 2 x 2 Ghz, 6 GB Ram.
Leopard 10.5.x