Actually, rather than a ballast, this problem is usually caused by bad end caps. I've experienced this several times. The first time, I thought it was a ballast issue. Bought a new ballast and replaced what I thought was a bad ballast, to only have the problem persist. When an end cap is going bad, it may not show up, until you've changed bulbs, disturbing the faulty end cap. On your fixture, I'm not sure of the procedure, but on Sunlight Supply fixtures, changing out the end caps is very easy.