FUSEBOX SOLDERING IS THE ANSWER!!!!!!!
I had a 73 240-- always had the speed dependent lighting system you discribe. Also the gas gauge was doing interesting things, like going to full whenever I stepped on the brake. It all went away with a new fusebox. All was right with the world. Until it came back this past fall. Hmmmmmm, new fuse box, all the connections were dielectric coated three years ago-took em apart and there was no corrosion. But my new nissan fusebox ($80 from the dealer three years ago!) was showing a little tell tale melting at the usual spots. Melting means heat. Heat comes from resistance...
Pulled the box apart, soldered all the fuse taps together, along with all the pigtails to the wires and connectors. Reinstalled it and WOAH!!!
I had really bright lights, that don't flare with engine speed, and my gas gage works once again. Hope this helps. Now whenever I get a car, I pull the fusebox and perform this before troubleshooting anything else. Its really amazing.> I have the same problem several other Z