My 68 Charger had the same problem. I changed out everything (new voltage regulator, new wiring harness, checked all grounds and connections, etc., etc., etc). It was so annoying that I had to have my idle set faster just so I could see when I stopped at a stoplight. I never did track down the problem and it actually factored into my decison to sell the car. Looking back, the only thing I didn't change was the alternator. After my frustration, that's where I would start
