Yes! I got it to work yesterday!
Same computer, same ProTools version (SE 8.0.3), same gear (Fast Track C600) same everything... And it runs perfectly!
The trick was... erm... to vaccuum the computer
Apparently the computer just got way to hot and the CPU clocked down to a lower clock frequence.
I made a song this evening with 8 audio tracks, 1 midi track, 1 drum loop, with 1-3 effects added on all tracks. And the system usage shows a CPU effects load of about 30%. Nice!