Aircraft Magnet Service of Seattle has a handout entitled "is it the mag?", in it he relates that most "bad mag checks" are sparkplug related. Per this handout: pull the plugs & test the resistance through them with an ohmeter. Stick one probe down inside the barrel & touch the other one to the center electrode. A new plug will read in the neighborhood of 1000 ohms. Anything over 5,000 is no good. Don't trust those "bomb testers", the plug can test good on those & still be out of spec on the resistance. Take my word for it, plugs can look just great with very little erosion on the business end & still be no good (been there, done that). Common wisdom sez to expect about 500 hours out of a set of plugs before they need to be replaced. BTW in my experience a Bendix mag will continue to spark a funky plug long after a Slick will not.
I know a guy who's C172 was running rough, his mechanic told him it was the mags & sent them off for overhaul ($$$). Got them back & installed and guess what-- it still ran bad. Mechanic told him "I guess it needs plugs too" and a new set of plugs later (more $$) it ran just fine. How about that! I already had a rather low opinion of his mechanic & this incident just reinforced that.
minimums are for busting