Apple Logic Pro /LogicExpress Discussion

I tried posting this message on the logic user group, but without success.
I recently upgraded from Logic Gold 6 to Logic Pro 7. As I play an arrangement,
occasional noise wil occur. This does not only happen at one specific time/place in my
arrangement, (it is not feeding nor is there a too high signal) and as I go back to the part
with the noise, it might repeat itself and then suddenly disappear !? :(( It is not my
speakers, mixer or amp as I have disconnected those and listened through my EMI 
soundcard AND the internal soundcard). The noisy parts and skips still occur with both 
audiocards. Also when I bounce both offline and in realtime my bounced file skips at 
certain parts and comes out with noise at other times. I can only bounce in LP7, a warning 
appears when trying to bounce i Logic 6.3.1. ("Warning: the selected environment objects 
have an unknown or illegal format. Please delete and create them again. Exit") It is then
impossible to press exit. The warning keeps appearing, and I have to force quit.
There are no skips when listening to my arrangement in Logic. Only noisy parts.
any ideas as to why ?
And then there are the noisy parts and skips when bouncing in LP7 - does anybody
know why ?

I bounce in AIFF, 16 bit, 44100 (CD-DA), Interleaved, with surround bounce and
dithering off, and I add to both my audio window and Itunes.

My last question is regarding system overload. As I press play, my
arrangement starts, and shortly after, a message pops up indicating that:
"CORE AUDIO: System overload. The audio engine was not able to process all
required data in time (-10011)"
Why is this ?

I use Logic Pro 7 with the newest drivers, my emagic A6/2, a Mac G4 867 Mhz
processor and 1024 RAM, I have three harddisks. One for my system (OSX 10.2.8 and
applications) 80 G, another one only for audiofiles from my audiowindow (150 G) and the 
third is for my ESX24 mk II samples (120 G).

Can anybody help as I am getting desperate here....

Thank you.
Sarah...