Yeah, that's weird. My only guess is that before, you had poor / dirty / oxidized ground connection(s), and that in working on the various chassis ground connections, you restored good connection somewhere. Dynamic load current, returning through a high resistance path would essentially modulate the battery voltage to control circuits, acting like a noisy, spikey voltage source. I had that on a Lexus GS300, driving me nuts for a year, replaced all kinds of parts, including the ECM (nightmare). In a last desperate act, I cleaned the several wire harness to chassis ground points, and viola - fixed. Should have done that 1st. An old lesson, learned yet again.
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