Make certain that you bring the faceplate screws up to torque gradually and evenly and be certain that the gaps on both sides are even. The suggestion of using Vibra-Tite or similar is a good one. Another thing to check is that the bars are actually 31.8mm and the stem is actually for that size bar and is not damaged or distorted.
I prefer a 4-bolt faceplate because it does not have a single point of failure like the 2-bolt kind. Since you have one why not just try it?