quadrature measurement noise

 
  Hi All,
   
  I have a strange problem using quadrature in comedi: not so random noise. The measure at a constant rotation rate is almost a straight line. When taken a difference between two consecutive points for speed, it exhibits some error for less than 1% of the points. For these points the velocity can vary up to 20% of speed value, while the rest of the points are well on target.
   
  I am using a US digital encoder 512 pulse per rotation. I also tried a totally different encoder with similar result. The axis rotation rate is 700 rpm. The board is a Sensoray 526 PC104. Software is Linux 2.6.17, RTAI 3.4 test1 and comedi-0.7.73 and comedilib-0.7.72. 
   
  Any suggestions are welcome, :-D
   
  Everett