It could be a low battery if it does NOT have sound.
Does your saber have lite sound? If so this is normal. I have a saber with lite sound a it has had a slight flicker from day one but I have not had any problems with it at all in the 3yrs that I have owned it and it has seen a fair amount of dueling.
My sabers with V3 sound do not flicker. Hope this is helpful. MTFBWY!
So, like the momentary power draw of power when yours generates sound it's slightly and visibly diminishing the brightness? Or at least that's seemingly a possibility.