EXS 24 Logic Sampler Users Group

dear group,
i just upgraded to logic 5.5 and exs 24 mk II - but now my system 
is getting timing-problems - syncronising audio and exs - the exs 
is getting out of timing and is playing the sounds to late - the 
same problems appear with any audio instrument i am tryiing to 
play - one exs instrument is fine - opening another sometimes 
ok sometimes not - opening a third instrument and everything is 
getting out of regular timing - any tips for me ?
system G4 733 - 1,5 GB Ram - Logic 5.5 - 2 x Motu 2408 - Midi 
AMT 8 - regards pete