We have installed a total of eight art lights. these are wall mount with (2)20w T3 G4 12v Halogen bulbs each. They come with a cord and plug in transformer that can be removed and be hard wired using individual transformers. These are 120V primary/12V secondary 75W transformers on each of eight fixtures. Alomost over night all but 2 bulbs burned out after a few weeks of being in place. We replaced all the bulbs and now the bulbs are out again after just a few weeks. The actual voltage coming from the transformers at the fixtures is 13V. Need some kind of idea of why these bulbs are burning out so fast. Any help would be great thanks.