One question. Does the frequency of the flicker vary with engine speed, or is it constant? Does it still happen when you are sitting still with engine running. Does it happen with engine stopped?

I don't think a failing diode in the alt. will always show on an alt. test until it goes all the way, except as a slightly reduced output, which is hard to test for on an auto parts store tester with a dummy operator.

Not responsible for advice not taken...