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because it likes to wink at you every time you start her up.
my best guess would be that the head light is leaking air some where and the vacuum from your engine corrects the issue some how. I know that if you pull the head light switch under neath that it opens up the line and that will pop the head lights up so it would seem that there is a leak that gets corrected or over powered buy the constant vacuum of a running engine.
get a diagram start pulling hoses one by one and vacuum check them. also put your finger over the actuator vacuum tubes and make sure that it wont move with either hole plugged. good luck