Too much voltage to headlights?
I cannot imagine what is going on, unless you have some voltage surge for a short period You could run 15.5--16 and it likely would not shorten bulb life by very much
Bumpy/ washboard road?
SURE?? the lights are rigidly mounted? They must NOT rattle and bounce
Any indication of surge? Weird "lights brighten" some time?
I'd be hookin up a voltmeter to monitor supply voltage for awhile, night and day, to see if it's doing something weird
What do you have for an alternator/ regulator? I mean, is it stock pre-70 stuff, or have you swapped "something else" in there or what?
Maybe you just got some "old stock."