Security Lighting System Fault Tracing

We have street lighting systems from one base to another in our particular camp , those street lights were arranged in series with one another by using different wooden poles .It had been observed that ,some of light were seen to blink and sometimes turn off for 1min before glow intermittently.AFTER fault tracing we decided to replace those middle light althrough the rest we are surely Ok but problem happen again .So could someone tell us what hero with our street light, cause the starting and the endling light both of them are OK.