Help - Top layers drifting/jumping horizontally?

I'm encountering a problem with Motion.

As my projects reach a certain level of complexity, there seems to be a threshold at which all further layers added above a point will jump at random to the left and back.

This skip occurs in the preview and upon export. Building a ram preview, clearing, and rebuilding it randomly recreates jumps at different intervals.

It doesn't happen with the projects' entire contents, only uppermost objects.

Here's a demo of my problem. The blue flash frames denote the upping of the birthrate on a particle generator (starting at 3, going to 6 then 12). As the system gets more dense, the jumping gets more out of control:

[rapidshare.de]

Again, this jumping can happen to anything. Mattes, video layers, etc. It's always horizontally. Always drifting leftwards and snapping back.

Can anybody shed some light on this problem?

My tech specs:
iMac G5 2gHz
1.5gb of RAM
Radeon 9600 128mb of VRAM
OS 10.4.4
Motion 2.0.1
Quicktime 7.0.4

I know I'm not running a very powerful system, but the problems I'm having seem more softwarey than hardwarey. I can grind my system to a hault with complicated filters without it skipping, but if I layer up too much, Motion can start skipping even while maintaining a high framerate.