Rewire your High Pressure Sodium fixture across 230 VAC instead of 120 VAC.
I'm presuming it has a CWA ( Constant Wattage Autoformer ) with multiple taps.
I also suspect that the wire gauge used to feed it needs 240 VAC.
You've made no mention of the distance to the outbuilding.
Check if the photo-eye can be replaced with a 230 VAC sensor, too.
Every time its voltage pulls low it changes its logical state -- which is the primary cause for the HPS to cut out.
