Pauls suggestion is a good one. My 66 is doing the same exact thing. Either that or one has it's brights on and the other doesn't which is what I thought it was at first but when I push the brights on the bright one goes a tad bit brighter and opens a LITTLE bit more but hardly can tell. I still haven't fixed the problem and may wait until I get a new wiring harness since the wires are eaten up and sometimes the headlights go out.