I think it's a mechanical problem, but I haven't confirmed that yet.
I just replaced an actuator on my S85 and had grand plans to take the case cover/electronics from my old actuator and install it on a new housing/motor/gearset and see if the problem was gone. But, when the time came to actually replace it, I chickened out as it's my daily driver and I don't have as much spare time as I would like.
This would tell us if the problem is in the electronics or electromechanical side of the actuator. Hopefully someone else will try this soon before I get a chance to.
I'm building a CAN sniffer/logger to try and capture the data between the DME and actuators. I just updated my post over on M5board with pictures of it. This will let me log and hopefully decipher the signals and then the Arduino program can be modified to allow it to control the actuator directly for bench testing.
If you replace one, take the old one apart and see what you find. I bet you will see that the worn spot on the gears corresponds with where the linkage arm is when the engine between idle and low throttle and the actuator has the throttle bodies fully shut.
If the S65 is like the S85, the DME commands the idle actuators to open fully first and once they (it for S65, we have two of them) are maxed out, the throttle actuators start to open up.
This leads me to believe that the gear wear may be due to chatter and/or vibration from the gears when they are not under load. I believe it may be a lack of tension that allows the vibration to wear the teeth.
It is interesting to note that in the picture above, the S65 actuator arms are *almost* vertical with the throttle bodies closed. On the S85, they are *exactly* vertical at idle.
I am curious from someone who has replaced one on an M3 if the actuator arm has to be moved from its endpoint to connect the ball linkage.