+1 on using a starter for saisons... I haven't had a problem with a stuck ferment using starters, and also making sure that there's a good amount of cane sugar in the recipe (e.g., 9-10%). I've also often made starters using two different types of saison yeasts (e.g., WLP 565 and WLP 568, or blending Wyeast and WLP). Don't know if that also helps prevent the stuck ferment.
Also, have you kept it at 80 consistently, without much temperature fluctuation? If the temp goes up and down, that can also mess with the fermentation.

Kim