1998 mercury mystique, 6cyl., keeps shorting spark plug electrode on cyl. # 3.?

1998 mercury mystique, 6cyl., keeps shorting spark plug electrode on cyl. # 3. Throwing 0103 code on same cylinder/misfiring cylinder # 3. I replaced the pug which, when I removed it, the plug had no electrode, but no signs of carbon or burning at all. It was just not there as tho it instantly cooked! I replaced it, the car idled nicely, drove great, more power etc. for about 2o minutes then whamo! started having a 'flat spot' during acceleration, running a bit sluggish but smoothly depending on the speed. n other words it runs as good as any car on 5 out of six. I'm thinking it may be either a short/ground interference with a wire rubbing against it that comes from the dist., or a bad injector coil causing a 'short' killing the plug. Any ideas?