This is a tattle tail on a high pressure sodium lamp that it is due for replacement,but you said you did that.Is it the correct lamp for the ballast,are the taps set correctly,maybe this is a bad bunch of lamps.?Will this fixture run fine from another power source?Is the photo cell facing the light causing it to jack on and off?These are some of the most highly efficient types of light next to the low pressure sodium lamp.I can't imagine voltage drop being the problem,unless it is being caused by a bad connection.
