A symptom of too large of a gap is sudden power loss at high rpm's. The spark gets blown out, like blowing out a candle. Proper gapping also depends on what modifications you have, if any.
I had my Autolites gapped at .035 and they kept blowing out when i was getting dyno'd (many engine mods). No one knew why. 3 mechanics/tuners couldn't figure it out. Then someone in this forum suggested decreasing the gap - hallelujah!