Problem using Cubase with iMic, sound doesn't play right Options

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I recently bought an iMic for my powerbook, but I encounterd problems using it with Cubase. I use :

- Powerbook Titanium G4/400 256 Mo
- OS 9.1
- Cubase VST 32/5
- an iMic, connected via USB

When the iMic is connected, the 'VST Performance' window shows a red light, to signal that the processor usage is too high. But the gauge only shows 5 to 30 %.
And when I try to play a song, no matter how many tracks there is or if there are effects applied, the sound doesn't play right : the sound is cut every 1 or 2 seconds, as if someone was playing with the MUTE button !

Does anyone know what could be the problem ?

Thanks,