HD3 not using resources efficiently. Advice??

[albee1952]

Got a simple session with around 36 audio tracks and 4 stereo AUX tracks.
All plugins are TDM(not a single RTAS plugin is in use)
Around 16 tracks are using sends to those AUX tracks for drum bus compression, room reverb, vocal reverb and delay(all plugins are stock plugins)
I keep getting playback stops with the error in screen shot and you can see the system resources window. It seems that things should be shared over the 3 cards, but it seems only the core card is actually handling anything and everything. Do I need to do something to force the system to spread the load over the 3 cards?
Session is on a separate 7200 rpm internal drive.
Session is 24 bit/48K
HD3 PCI cards in a Digi-branded expansion chassis
A single 192 IO is connected and all 16 analog inputs are working fine.

more system info:
G5 dual 2 GHz with 1.5GB RAM Leopard 10.5.8 and PT 8HD with license on gen 1 iLok.
500GB SSD system drive
160GB 7200 rpm drive for session(blank drive except for this session)
single 27" monitor @1920x1080(also tested with 24" monitor at several resolutions).
NOT connected to the internet for testing.
Tried copying the session to an external SCSI drive(15K rpm) but it was much worse)
HD3 PCI cards in Digidesign chassis(cards and 192 pass digitest).
Very new to Macs and HD systems(I have repaired disk permissions)
I am the admin and only account on this machine.