Hey, maybe dragging up an old thread can be helpful... Yep, this problem is a very common. I suffered with it for a couple years before I finally found out what it was (that was before the internet). I don't know if you caught it, but you might check this site which was referenced in a previous post: http://www.midnightdsigns.com/james/headlights.htm
mine did the same thing to me a wile back. i ust replaced the dimmer switch in the floor board was like 4 bucks and mine work great now no problems at all
If the lights are ok initially, but over time begin to flicker, getting progressively worse and off for longer periods of time, then this is probaby your problem.
My '67 did exactly this. I replaced the switch after reading in M&F that the stock switch could not handle the extra draw of halogen lamps. Haven't had a problem since.