Oh now. ....light.sunShadows...
This sound like pssm shadows. Older pssm had black lines and shuttering effects. Slin and others worked them out better.
Just working the idea, could be you have to stop and restart pssm on resolution changes. Also adjust the shadow map resolution and split numbers.
Turning off any pssm sun based shadows will greatly improve rendering speeds. Slin's last version would cut my frame rate in half, so I'm sure shade-c pssm shadows do at the same.

Pssm was a bottle neck. It required fine tuning or you'd kill frame rates. I also know using stencil shadows cause black lines from unclosed mesh models.