Was the weather different when it was tuned vs. when it started sputtering? Possibly just a tad different, like 10-20*F cooler outside?
What are your plugs gapped at? Its very possible the gap is too large, causing misfiring under boost. I had this happen after going from a vf34 to a 16g.
There really is no "proper gapping," that can be assumed for all cars; it depends on one's setup.