Hi,
It sounds like you eliminated most of my possibilities, which is good. I was looking at my BME engine manual and it says to open the plug gap if the engine runs rough in mid range and close some if it misses at full throttle (gap .020" to .025" ). The plug used is a NGK BPMR6F which is a resister washerless plug. The nipple is removable so you can use what ever type nipple is required for your plug cap. AV8TOR thought that it may be a plug issue and it sounds like he is right. Let us know how it goes.

Dave