The reported fault was that the unit would energise the relay and load when powered, but would then drop out after a second or two and start again. That's often a common sign that a power supply is failing to deliver enough current. The circuitry runs until it tries to energise the relay, which then pulls the voltage down low and drops out or resets the circuitry.
In this case I was able to diagnose and repair the unit, but the original components were very small and the module is a close fit in the case. I'll order new components for it.
The reference to using an external USB supply is one of those emergency fixes that can get you out of a hole when something fails and you can't get an immediate replacement.
To elaborate on why a failing capacitor can cause intermittent crashing of an older piece of equipment, especially if it runs hot, the capacitors characteristics can change giving it the appearance of being a much lower value. The fault is hidden until the increasingly rough DC finally gets low enough to be below the regulation level of the voltage regulator and the voltage dips in the circuitry supply can cause progressively more crash prone or glitchy performance. Capacitors are cheap enough to routinely change out to rule them out as the possible cause of a problem.
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