PTF audio & Cubase MIDI?

[edjog]

Got no joy posting this in SadRobot's thread, so starting a new 1!
I have tried using IAC Bus with PTF and Cubase.
1st problem: PTF failed to open with a 227 error. Solution, close Cubase, open PTF, then open Cubase.
Next thing is wierd: I disabled the audio engine in Cubase because I only wanted it for MIDI and set it to recieve MTC from IAC Bus. Set PTF to generate MTC to IAC Bus, sorted I thought, synched playback!
Except that I could only get audio from channels 1&2 in PTF. Even quitting Cubase and then quitting everything and restarting the iMac did not help. I returned every thing back to normal in that session and still: only 2 channels of playback. Any ideas how I can get 8 channels of audio in PTF and full use of MIDI in Cubase, at the same time?
Computer= iMac G3 500MHz, 512 MB RAM