It amazes me how such a simple install can be so messed up. In the past five years, I personaly have installed over 700 lowbay and highbay fixtures (matal halide ranging from 250-400w) and NEVER heard of fusing the individual lights or had a problem. 1 20a 277v breaker with eight lights should be fine. And I think you said there is two rows of four, each row on a 20 a breaker. Should not share a neutral either. There is no point. Could share a ground. How are these lights controlled? Is it a standard 277 rated switch, or a contactor? Sounds to me like it was a poor installation to begin with and you have bad connections. Plus some of the fixtures have been energized, but the lamp is not lit, which will kill a light eventually.
