My TSIO 550 with GAMI injectors, Slick Mags has backfired 4 times
recently.  In cruise at FL230-250, 2400 rpm, MP 29", 70 degrees LOP.
Quite unnerving.  I suspected a spark plug or wire.  I flew it to 8000',
29", 2400 rpm, and LOP, then right mag and left mag. I repeated it at
2500 rpm and 31", LOP  (I never run at this power setting LOP) The
engine did not backfire or miss as I would expect with a bad Plug or
wire.  The right mag ran a little bit rough.  The left mag was smooth.
Any suggestion what to do next?  I am committed to aggressively pursue
this.  

Carl Cadwell, IVP, 235 hours.