Kanga has a good point. Most factory strings are made with inferior workmanship and thus, the keep on creeping (elongating--most say stretch). Some eventually settle down, some don't. God aftermarket strings is the best answer. With all the ones I've used (Winner's Choice, Prostrings, Vapor Trail, Gibblet- aka Bucknasty) you install them, shoot 50 shots or so and then install the peep. After that they rarely creep and no peep rotation. Of all these I like Bucknasty the best for quality and price.

Something else you can try is what Texas said above. Don't get the idea that a string loop is used for aligning a peep. It can help, but is just a bandaid for an underlying problem. This is not what a string loop is for. You can add or subtract twists from a string to reloacte the static position of the peep so i tolls around into position at full draw. You can further fine tune this by moving string strands from one side to the other. Doing this be certain to have the same amount of strands on each side of the peep to balance things out.

Push comes to shove, you're too cheap to buy good strings, or whatever reason you can always use a peep with a tube aligner. Again, just a bandaid for the underlying problem, but it works.