Lighting problem

Orbit fixtures do not utilize starters. I would zero in first on the
contacts of the sockets / bulb and any wire connections..but mainly
the actual socket connection that contacts the bulbs contacts. I had
similar problems and a good cleaning fixed it. Even a little salt
creep does strange things. Then get a meter and do some voltage /
resistence readings. Could be a ballast, but could be a simple thing
like corroded socket / bulb contacts. Don't take it for granted that
a new bulb is free of oxide or corrosion,as I have purchased bulbs
that had a oxide film on them already and gave problems until the
contact pins were cleaned.