Just finished putting my '67 RS back together. The headlight doors open and close correctly, turn signals flash, hazards ok, everything seemed ok. My body man took it to check out a door alignment situation (in the evening) and noted this situation: With the headlights ON, using a turn signal, makes the tachometer "beat" with the turn signal. In other words, as the flasher flashes, the tach drops to 400 then back up to 1000, then to 400 and back and forth. As a test, a "smack" to the top of the dash made it ok again for a while. Does this sound like a grounding problem to you guys? Or what?