machfive on G5 : performance issues

[bbrichau]

i have bought machfive
i am running it within dp 4.61 on my imac g5 2ghz 1.5gb ram os 10.4.5
i upgraded to version 1.2.3 of machfive
when i load the bosendorfer piano and i want to play 12 notes at the same time (12 note polyphony) only 1 note plays
i have to increase polyphony to 127 for it to play all 12 notes...
also, the cpu meter in the audio performance window of dp 4.6 blows out in the red all the time and the sound crack and clicks and pops
please help
ps : no other applications are running and I have killed dashboard