Your symptoms are the ones normally given by a bad/dying bulb but you've replaced that, and the voltage readings are ok. In my job we maintain 51 schools and there are hundreds of these lights. A long time ago we realized that it just isn't worth the time and trouble to search thru all the pieces to find the bad one. (and yes, we have the meters to test each component separately). Half the time you have to special oreder the individual component after you identify it. We find it quicker and cheaper to just replace the whole dern innerds of the fixture and not spend the time troubleshooting them.
