You didn't mention which level your mages are, but if you have Ring of Frost, you can drop one down while you run.

If your mages are arcane, use Presence of Mind, for instant sheeps.

or remove stopcasting (it seems to be a tad unnecessary there) and spam your sheep button if one sheep breaks early.

If you want sheeps last for full duration, check your hit rating, you need to be hit capped against the mob lvl, so they don't break early.