[Elecraft] strange tuning effect on k2

[Carel, pc5m]

Hi,  busy building a k2 and have following problem:
 
- Alignment and test part II
 
Performed this alignment (a.o. VCO alignment and VFO linearization),
everthing went well, but:
 
It turned out that that when tuning the frequency the tuning is very
strange, no 10 Hz steps but rather erratic but after every 5-10 KHz it goes
ok. Display gives the idea everything is ok (moves very smoothly) but when
listening with a seperate receiver to the internal RX vfo it is not smooth.
 
Measuring the PLL reference frequency Xtal VCXO: not smooth
Measuring the PLL reference voltage out of the DAC: voltage is jumping !
 
Strange is when I perform the VFO linearization the DAC voltage is very
smooth progressing and also the PLL reference is moving very smoothly. VFO
linearization ends with "end" no error. 
 
Anybody an idea what is going wrong ?
 
thanks, 
 
Carel, pc5m