Matrox workstation only using 2 of 8 cores!!

I have a new dual-quad core workstation with a Matrox RT.X2 board installed using Premiere CS3 (see profile for more detail). I'm trying to do a project using stills in a move and scale manner over a single track of DV video footage. According to replies I've gotten in the Matrox forum, it can't handle images larger than a standard DV frame (720x480) without rendering first!

Therefore, I'd like to bypass the Matrox hardware and simply use the system CPUs to get the job done. But, when I use Premiere's scale and position settings rather than Matrox's filters, I only get use of 15% of the cores and the video stutters / drops frames as it plays from the timeline.

As far as I can tell, the latest drivers are properly installed (Matrox 3.1?). The hard drive array passed the benchmarking tests with flying colors and is defragged. The standard video card's driver is current as well. Any ideas?

- David