Well, intermittent problems can be very hard to find. The first thing I would do is take out one of your headlights and hook up a voltmeter to the low beam connections. I would expect to see some where between 12-14 VDC. If you get a lower voltage make sure your light sensor in the dash is covered.

Before I get to deep in this, is your car completely stock? Because if you have done anything to it you better speak up now Many people neglect to mention mods, because they don’t think they are related to their problem. Did you buy the car new? If not, any sign some one else mod’ed the car? I ask these questions because I had a car years ago with headlight issues, and the cause was a security system that had been ripped out the car by some idiot. Also, has something else weird happened to your car, like get doused down with water?