I think Wrenchbender is right.
I'd recommend that you invest $10 or so in a cheap voltmeter, the kind with two leads and a read-out to tell you if you are getting full power to each of those slots in the socket or not. We've used the light-style testers too, but when you are chasing a problem, it is nice to get real numbers.
Trace your full circuit. Don't trust that your black is necessarily your ground. After all these years a lot of patches get made in the wiring harness. You might want to pull out your other headlight and verify they are wired the same.
Maybe one of the other guys (who are smarter than me anyway) have another suggestion, but I'm betting on the ground.
K
