KONTAKT 4 audio dropouts on MacBook

Hi!

...apologies if this has been discussed, but my problem appears
in so many different permutations that it's hard for a newbie to
see the wood for the trees.

I've got a very straightforward setup. I have a normal MacBook
in a pretty out-of-the box setup. (normal white plastic MacBook,
not the Pro or Air; bought it in May 2010; 2GB 1067MHz DDR3 RAM;
2.26 GHz Intel Core 2 Duo; Mac OS X 10.6.3). The only possible
problem I see with my machine is that the disk is quite full, but
probably not wildly fragmented, i.e. I had plenty of space
yesterday, but then I installed KOMPLETE 7, and now 221GB
out of 249GB are used).

I configured KONTAKT 4 with the normal 512 samples buffer,
and I'm having it output to the normal built-in audio device.

When I use the Vienna concert grand (which was my main
motivation for getting the whole rig in the first place), I hit
the sustain pedal, and play some of the low keys as fast as
I can: That will give me some really ugly audio dropout
artefacts, where the samples kind of tear off.

What should be some of the first things I should look into?

I noticed that whenever I hit any key, the label "DISK" lights
up, but it doesn't show any utilization. I've also watched it in
activity monitor and found that some sample files are getting
closed and re-opened while I'm actually playing. Are the
samples not actually getting loaded into memory and accessed
from there? (i.e. is disk speed an issue at all?)

My first intuition was to try and put the sample files for that
instrument on a ramdisk, but shouldn't the OS's disk cache
make that somewhat superfluous? (it's always the same two
files getting accessed during my testcase)

What can I do to upgrade my MacBook hardware-wise that
might help? Would more memory help at all?

Also, I'm trying to understand more generally what happens
when I use KONTAKT as a plugin for downsampling, so I then
set "offline interpolation quality" to "perfect" and configured
Sibelius to use KONTAKT as an instrument. I then recorded
my frantic test-playing into Sibelius which, in realtime produced
artefacts. I then used the "Export Audio" functionality of
Sibelius and, in the downsampled audio file, I could still hear
the artefacts. Any ideas what might be going on there?

Thanks a lot! Any help would be much appreciated.