Buy a new ballast for one, install it and see if there is any change,
for the good, in that light. If so, change the ballasts. If not,
then it's probably a ground issue.
Replacing the fixtures, maybe? I bought seven 4-bulb fixtures (T-8
bulbs) for $35 a piece at Lowes. I didn't think that was a high
price. I got one "damaged" fixture (broken bulb only) for $10.
Check for damaged fixtures for discount purchasing.... at Home Depot,