Its a very weird and confusing problem,indeed.
This one would be difficult to remote-diagnose I guess.So....um...lets see.What'd I do if I were in your place? I'd try to figure out the root of the problem.
-I'd reinstall ELS to see if it solves the problem (keep a backup of your ini,of course)
-If that doesnt help,I'd switch my version (if I were to use v5,I'd use v6 and vice versa--just to check)
-I'd look for glitches in other lights (civilian cars,streetlights,redlights-if they have some bug--again,just to check if their were any suspicious irregularities which could indicate the problem to possibly exist in light shaders/occluders)
-If that doesn't help,I'd restore my backup ( always take backups when my game is in a perfect running position).