Hi,
I did not go into detail with your tests. But there are some general rules for testing. I´m not sure whether you followed them:
* ESD protextion: DUT, power supplies, measurement devices, desk surface, and operator need to be protected against ESD. This is not only unintentional static voltage, but also the voltage that is generated inside the power supplies with respect to EARTH GND.
* no (unused) input must be left floating.
Also I recommend to properly bypass every power supply close at the DUT.
Klaus