streetlights cycling -- layman's inquiry

The HID HPS luminaires in the downtown are cycling. I am a taxpayer
activist.
The contractor is either baffled or enjoys replacing bulbs every 6-8
weeks.
All the affected luminaires have an underground feed and were
installed in 1998.
If one luminaire on a circuit starts the others soon follow.
Since contractor's original installation, most bulbs lasted less than
a year.
The contract calls for payment for each and every service call.
Contractor says ballasts are fine.  I suspect not.

Is it possible a line problem could be at fault? 
If it's a possibility, would the remedy be expensive, i.e., involve
tearing up the street ?  Is anyone familiar with contractors being in
charge of city lighting ?


Any experience and insight appreciated.


Tom Beckett
London, Canada