my guess is the turn signal switch in the steering column... I'm guessing that it doesn't happen during the day because your high beams can only turn on when you pull it and not push it. (unless the lights are on)
So thats what i worked out in my head. get what i am saying? i guess its one thing to check....
well how about this... while the lights are on and you switch on your turn signal, try wiggling the turn signal stick to see if it is doing a weird connection if the lights go back to low. Pull it back a little bit maybe... forward... i don't know. Let us know what happens.