Hi Gar, Your grounds are fine or the lights wouldn't come on at all. You have to have +12V applied for the highs to come on. Your troubleshooting points to the stalk. It sounds physical, maybe a terminal inside has broken loose intermitently making connection. If a wire is off and touching, I don't think you could get it to turn off so predictably. the highs should come on flacky when you flick the highs on with the stalk. I took a look at the diagram, the brights work with the headlight switch off. The stalk switch goes directly to the high side of one of 2 fuses #9 and 10. If either fuse is blown only drivers side high comes on. strange to me. The stalk applies B+ directly to the back side of #10 fuse. I do see the dash bright light should be on too. And the floor switch is between the light switch and the supply or input side of the 2 fuses.
Anyway, pull the stalk switch and see what's a miss.