Leaf curl towards end of flower

[Fastcar]

I get leaf curl on all strains with about 2-3 weeks to go of flower. I lower ec slightly before flush but everything else is the same i.e temp
Any ideas what this could be?
Thanks

 

[The Germinator]

Pictures please.

 

[RadaRoast]

Could be numerous things. My guess just from the sounds of it is Nitrogen Toxicity but cant be sure. Different kinds of curling means different issues. Pictures please.