[PD] pd/arduino pwm > servo motor

[tim]

I have a servo motor which is to be driven by pduino / Arduino.
The following firmware directly uploaded to the arduino (near bottom of 
page):
http://itp.nyu.edu/physcomp/Labs/Servo
runs well.
With some modifications the motor moves between 2 positions smoothly enough.
However, trying the same from pd using the pduino (fermata) firmware and 
the arduino help pd patch, it's quite a different story.
The pulse-train needed for the PWM control of the servo motor seems too 
irregular (due to pd/comport timing?) such that the motor is not 
positioned cleanly but jumps around. (at least, that's my interpretation)
I tried both sending PWM from ports 9,10 or 11 (the Amtel-built-in pwm) 
and generating some software pwm on other digital out ports.
Is there a way to get rid of this jitter?
( last attempts were running pd 0.39.2 on ubuntu, comport from: 
http://wikihost/wikis/techartsound/var/gebo/data/file/comport.pd_linux )

thanks,
Tim