I had really weird light problems on our 70 RR when we first got it. I had made a new rear wire harness to replace the hacked up one. I finally had everything hooked up and it had been working perfectly a couple of days before. I brought Michelle down to show her the now finally working lights and I had very similar problems. With advice from on here I realized it was a ground problems, as has been suggested for you (and I agree). I ran a ground wire from each taillight housing to the trunk latch support and at least THAT part of the electrical system has worked fine ever since. Yes, I know it is red not black wire but that is what I had. Good quality tinned marine wire with heat shrink ring connectors.