Red cocker valve spring, cut down Spyder main spring. It should work wonders for you. If you want to run lower pressures, turn an "hourglass" shape into your valve stem, and enlarge the passages, is what I've heard. Supposedly someone got a Sterling running WELL at ~500psi this way.
Mine is an oldie, running a vertical GX-2 reg. I think around 600 psi, with the springing I've got in it.