Fluorescent bulbs keep burning out.?

I have 9 - 4 foot, two tube shoplightes in my woodworking shop. I have a problem of tubes burning out in a few weeks / months in three of them. It is always the same fixtures and always the same side/bulb in that fixture. Others have been in place for years with out having to change the bulbs. I have also come across this issue in some homes that I service. Obviously this is an issue with the light fixture but what is the problem and can I fix it without replacing the fixture? I have tried a range of bulb brands and price range with the same results. All of the fixtures are on the same elect circut controled by the same switch.