Remove the top part of the plastic panel covering the steering column between the wheel and the instrument cluster - 2 Philips screws. You'll see a short Brown wire coming out of the switch assembly. Note the ring terminal on its other end. Tighten the small screw that fastens it to the die-cast switch(s) mounting bracket. This wire provides the ground that the switch needs to trigger the high-low beam relay.

If that doesn't cure things, try again spraying some lube into the switch workings while they are exposed. (This is not a cure, it's a workaround. The cure is a replacement assembly from the PnP, $4.99 here - and they don't care if it's the 'cruise' lever.)

If that doesn't help, remove the wiring plugs from the below mentioned high-low relay, pull the relay and check for corrosion (greenish crud) on all male and female connectors. That 58a terminal on the relay has to carry the full current to power the 130-watt demand of the high beams and it's a rather small rascal, so needs to be clean and tight.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD and 89 745 (LT-1 V8); hobbycar 77 MGB, and a few old motorcycles)