Ballast/lighting question

[kien]

About a year ago when I first got my light fixture and ballast I had this happen. They were brand new ballasts (galaxy) and the first one would only run for 4 hours then would shut down. Returned it for another brand new one and the second one would run for 6 hours then shut down. The second one did this a few times before it eventually stopped firing all together. Wasn't the bulbs because they were brand new and I swapped them around to test. Third time was the charm as they say. Been running my third replacement ballast ever since.