All lights flicker when running

2x on bad diode or two. The alternator is a 3 phase unit, loss of one part in phase results in dip, the regulator responds, but is too late, so over voltage with next phase...regulator then see dip.... bumpy voltage, hence flashing lights.

A scope helps trouble shoot that problem.
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The diodes flip the negative section, so the the result is a series of humps. Imagine a missing hump, or complete phase loss. The dip, is not corrected by regulator correctly because the next hump is good, that is where over voltage happens.

The earlier mechanical regulator exhibit more flicker. The electronic regulators are faster, and may mask diode problems, and a few are driving with bad diodes and do not notice.