AD9837 Error state by frequency sweeping

Hello!
My AD9837 goes into an error state by doing frequency sweeps. The problem occurs at random frequencies and sometimes it passes more sweeps without interruption.
I am not doing the sweep like described in AN-1044.pdf but with setting each frequency individualy every 20ms. I am controlling the DDS with a PIC Microcintroller and the reference clock is 4Mhz.
When the DDS is in the error state i have to restart it completly by disconecting it off power and wait for some while and after that it works.again until the next error state.
I can't find out how to solve that problem or what could be the cause for it.

This is how the error state looks like: