Decades ago we had (switch mode) power supply problems on the new fangled Apple II computers.
Turns out there was an electrolytic coupling capacitor that provided PWM feedback to the power stage through a transformer (rather than an optocoupler).
The capacitor was forward biassed under light load, but we were a research lab and we filled all the available slots with cards. But under this heavy load condition, the coupling capacitor was reverse biassed and would eventually depolarise.
Swapped the cap for a bipolar type and it fixed the problem.
Replay available now: A handful of emerging network technologies are competing to be the preferred wide-area connection for the Internet of Things. All claim lower costs and power use than cellular but none have wide deployment yet. Listen in as proponents of leading contenders make their case to be the metro or national IoT network of the future. Rick Merritt, EE Times Silicon Valley Bureau Chief, moderators this discussion. Join in and ask his guests questions.