quadature encoder

I have the following problem.
I use the BL2600 board.

To control my system I use two quadrature input (PIO18, PIO19) and (PIO20, PIO21) on the BL2600.
Counting freq. Is max 5860hz on each quadature input.

Without any reason the counter increase with 256.
For example
The counter stays on 10000 and by the next step of the encoder the counter can stay at 10256 without any reason instead of 10001.
I know the there is an 8 bits hardware counter on the BL2600. When the counter goes from 255 to zero the counter gives an interrupt with offset 0x0190.
It looks like that the counter gives unexpected interrupt.
This problem is a big problem for our system.