[RWebb]

maybe use an analog voltmeter to check - you tested under the same conditions as the when the flickering occurs?

next, I'd remove each ground and clean in a strong acetic acid mixture - I usually leave it overnight

I use an acetic acid saturated Q-tip or piece of paper towel held on the grounding point with tape - you can put plastic wrap over the area too & seal the edges

then hose it off with electrical contact cleaner and use dielectric grease when reassembling

was the car ever back East? West coast cars usually don't have ground problems to the extent that rust belt cars do