No, this is not the normal behavior but I had that exact same symptom at some point... I don't have that now, I was doing 'circles' with the trackpad for 10 secs + no jerk.

I don't know exactly what causes it. Is it that you need to boot with -s, remove the voodoo kext with rm- r (a bit like what you do to install the GMA) then reinstall the driver with the pkg or use the update procedure in iatkos to upgrade only that? (you'll need an external mouse at reboot if you try to go in the gui after removal).

I also though when that happened to remove speedstep, or use cpus=1. But at this point I use speedstep with cpus=1 and I don't have the problem. The voodoo.pkg installs the same thing as checking 'Voodoo with trackpad' during install but sometimes it yields different results. My intuition is that it's related to installing a driver on top of another one...

Seems you solved almost all your problems!