PTH12040 high output noise at certain voltages/currents

I'm evaluating PTH12040WAH and found strange behavior of output voltage. It shows relatively large voltage oscillation/fluctuations just at some output votlages & currents.

 

This output around 1.6V 4A. The signal is AC coupled to view only ripple. Also notice the time scale, it's not switching ripple but regulation ripple.

 

And this about 1.7V 4.5A

 

The filter capacitors are certainly sufficient (I have tried many different sizes & configurations but the result always same).

*Input caps are 2-3 pcs 470 uF 10mOhm ERS poly aluminium

*Output caps 1-3 pcs 1200 uF 10mOhm ERS poly aluminium

*Output load is two high current silicon diodes in series (just for testing)

*Voltage adjustment with trimpot

 

What could be wrong or is this normal?