5V 3A source for minicomputer using FBL2360

It sounds like you have poorly filtered step-down converters, which is typical of cheap Chinese parts sold on Amazon and eBay.

To make them better, you should add a 5W 5.1V Zener clamping diode (or TVS) across the output of the converter, as well as a 100 uF capacitor; this will protect against over-voltage shooting through the converter.

In addition, you should add a 100uH or better inductor with 3A or better rating in front of the input to the converter, and another 100uF to 1000uF capacitor after that inductor before the converter. This will filter the power rail before the converter, making it have an easier time keeping the voltage rail clean.