The plugs should be torqued to 30 ft/ lb and the threads should have a little graphite anti-gauling compound put on them before re- fitting.
The only thing is that I have yet to be convinced that it is a problem with the plugs, AVGAS has a compound in it to encourage the evaporation of the lead, this compound starts to work at the temperature developed at between 1600-1800 RPM. This leaves me doubting that it is lead fowling problem as it was reported the engine has been run at 2200 rpm and that would most likely have cleared the lead fowling, so may be it is oil fowling or a malfunction inside the Mag. A little more info on the scale of the mag drop would be helpful.