I have four 10 foot fluorescent utility lights in my shared garage. (2 bulbs each) 3 of the units no longer work properly even though I've replaced the bulbs. One of the units works sporadically and another only lights up one of the tubes. I don’t know if I need to replace the whole unit or is there a piece on the unit itself that needs to be replaced? Any suggestions would be greatly appreciated.

Certain fixtures have problems with certain bulbs. If the fixtures are old they could contain starters. The bulbs have changed in recent years and they are difficult to start on the older ballasts.

Inside of the fixture there is a transformer called a "florescent light ballast". They can be replaced with electronic ones or non-electronic ones. The big box stores like home depot even carry them. You do nned to know the bulb type and the number of bulbs.

You should put a little dialectric grease or vaselene on the electrical ends of the fixtures. This will help prevent corroson in damp environments.