Steve
I also use a 2408 but in "slave" mode. The 2408 can do some weird things with the PCI when it boots up, so I have found these steps good for diagnostics/troubleshooting:
I usually boot the d8b first (my master), play my keyboard to make sure all is good....
Than boot my 2408, play my keyboard again to make sure there are no no Clock issues....
Than turn on my G5 Mac and play my DAW.
Sometimes I need to boot my 2408 again if things freeze up. The PCI-424 card can cause some problems at times. This sequence seem to work best for troubleshooting.
See my config here viewtopic.php?f=2&t=609&start=40
and have a look at my thread as mentioned by Crash above as well as what synthjoe's comment.
Good luck