Sorry, I'm new here. Posted in wrong forum, but can't delete. Post moved here:
http://www.gamedev.net/topic/634071-deferred-shading-light-accumulation-issue/

I'm not sure if I understand the problem correctly, but several points came to my mind:

- have you disabled your z-test / z-writes for the lights?
- are you drawing the lights additively to the accumulation buffer? You'll need to set source and destination blend values to GL_ONE.

Cheers!