NI 6602 - comedi_data_read(..) takes 4ms (RTAI 3.3)

Hello people,

I am using:
* RTAI 3.3
* NI 6602  (For encoders only)
* PCI-DDA08/16  (for Analog output)

I'm running a single task (about 4KHz) that reads encoder values and ouputs
an analog signal.
When running for 20 seconds, there are usually a few instances ( 1 - 3)
where reading values
 from the encoder takes about 3800 - 4000 us. Which is, of course awful.
Without these, the
maximum is about 60us.
   I have disabled as much hardware as possible from the computer, but still
the problem remains.

Does anyone have any clue on what I should look at?
Hopefully this is a common issue with a common solution...

regards,
Camino