Sounds like you're pots are dirty Erwin, pull the cover off it, turn it upside down and spray contact cleaner into to pots, move the pots through their entire range a couple times and problem should be fixed.
Spray it into the gold casing where the three wires come out (not on the wires), one short squirt should do it.
The reason it's only happening when you turn the wheel to certain paces is because the wheel powers the pedals, when you turn the wheel the power drops, with clean pots it doesn't cause any problem but with dirty ones you get the pulsating effect.
Also consider a Bodnar box, what this does is turns the pedals into a second controller and they're powered from the USB port (we're talking tiny current so little spikes make a big difference), the box also changes the resolution of the pedals fro 256 steps to 1024 steps, giving you a lot more brake and throttle control.
http://www.leobodnar.com/products/DFPG25conn/