Our kitchen is lighted by two four-tube units. Years ago I went through a frustrating sequence of replacing tubes, not having them come on consistently, replacing the ballasts, having no better luck, and still going through a number of tubes before getting all the tubes to come on consistently. We've been ok until the last couple of weeks. Now both units, on separate switches, are very erratic about coming on when switched, sometimes delaying quite a bit. No other apparent electrical problems. Can anyone tell from that whether we should be able to just replace some tubes, are apt to need new ballasts, or what?