I have a fluorescent light fixture that uses two 40-watt 4' long tubes. It has recently started acting strange. When first turned on, it works fine for a minute or two, then one of the tubes becomes dim and the other becomes dim and flashing. It's not the tubes, as I've replaced those and nothing changed. Someone suggested it might be the ballast. I'm not exactly sure exactly how old the fixture is, but I'd guess it's at least 10 years old and has been used daily for that entire time. Is the problem caused by the ballast, and if so is that something that can be changed, or should I just buy a new fixture?