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last night, I pulled out my multi-meter and oscilloscope to make measurements on the Kiwi.
The Kiwi is mounted in an aluminium cabinet with no external access to the 5V Kiwi barrel connector, since the cabinet also contains a LM-350 adjustable voltage regulator module, with 5.3V output soldered directly to the 5V barrel connector pins on the Kiwi PCB.
Using the digital multi-meter, the power input to the Kiwi looked fine, 5.3V and stable.
Then I connected the the oscilloscope to the 5V input, and noticed that the input voltage to the Kiwi contained very deep voltage drops when vibrating the cabinet and/or touching the power wiring inside the cabinet. Finally I located the problem to a imperfect connection in a PCB screw terminal connector on the voltage regulator board. Fixing the connection and re-checking with the scope to verify solved the problem.
Lesson learned: Never trust a digital multi-meter alone when investigating problems with power supplies!