Are you calling "Burn out" the same as "Brown-out?...Clematis typically get ugly brown leaves during the hot part of the growing season especially when not watered enough..remember that clematis need to have their roots stay moist and maybe it's a sign that you are not giving it enough water during the summer...I would think that clematis in your temperate zone could take alot more sun than mine in hot Texas Sun..zone 8b..mine are in partial sun and I still must water...Jeanne