I purchased a 3-pack Inreda light system (http://www.ikea.com/au/en/catalog/products/50105330) which are halogen and they connect to some sort of transformer. When I connect these up at first they are very bright, but over time (7-10 hours of use they dim/yellow. Why does this happen?

I have tested them out with new bulbs and it doesn't help. However, when I tested it out with a new transformer (same exact one just unused) the ligths were much brighter. My guess is that this has something to do with the transformer. Can anyone explain why this happens or what I can do to remedy the situation?

Thanks in advance!