This is very, very similar to my experience, and I've probably pushed my kites in harder conditions during that same time.
As was mentioned, sleeving quality and braid may have a lot to do with it.
I tend to use line sets from The Kite Shoppe myself...
They use cored bridle line for sleeving by cutting it to length and pulling out the core, this leaves a very tough, tight braid sleeve behind that has worked the best for me.