Exactly what 54Bore said. If you have less expensive strings/cables you should shoot at least 100 shots to get the stretch out. Then once that is done you should check your axle to axle distance to see if it is within the manufacturer's specs and twist string as necessary.
Once all that is done and you can realign your peep. Also, you say that you adjusted your D loop to compensate but it only last 3 or 4 shots. Seems like your D loop knots are a bit loose. The best tool I've found for tightening a D loop is a set of snap ring pliers which you can pick up at Harbor Freight or Lowes.