If this is happening on more than one breaker (unless circuits are sharing neutral ) it probably has to do with panel , meter , or in between . As in my previous post , SE cable running across concrete product can corrode , eating through the outer neutral braid , causing dimming etc. If you can't check this , perhaps you can run a TEMPORARY parallel neutral from panel to outside , or measure for voltage drop on entrance cable . Also check if main breaker is loose connection in panel , ie voltage drop across breaker .