Make a waiting period on the bonus ranks gained. It takes a full week maybe for you to gain the skill points. Other magic items have a waiting period for your body to fully adjust to them. Ring of sustenance for example. This would remove the issue of Fox's cunning being used to gain skill points on a needed basis. Although, in most situations this spell isn't that powerful, so that would give more incentive to take the spell. I personally don't give skill points for Fox's cunning and have a house rule for skill points that is similar to the bonus spells granted. It simply doesn' take effect until you rest with it. This prevents the item from being passed around to use as a skill boost when needed.