Perhaps the runoff is because of too much peat moss, which can form a crustlike surface under or over the soil. For a rock garden, I would suggest adding loam to your sand and leaving it at that, because rock-garden plants prefer soil that is on the sandier side. Once your plants become established, their root systems will help hold the soil in place. Until then, you may need to rig up some sort of retaining system using additional rocks to keep the water and soil from washing down the slope. Mulching around the plants also will help keep the soil in place.