running powerbook g4 1ghz 512mb ram OSX 10.3 cubase SX 1.501 with standard core audio card in laptop using m-audio oxygen to trigger.

On a bass sample set, when certain notes are triggered the vst CPU usage spikes all of the sudden, its intermittent but quite annoying. do samples from kontakt get loaded into the ram? it doesnt spike the cpu all the time but when it does it crackles but when it doesnt it hovers at about 30% with 3 racks loaded into kontakt! idealy what should i have the buffer set at in the vst multitrack options? what else could be causing this? any help would be fantastic! thanks!