I have always been hesitant about checking mags in the air, but no more-or I learned about flying from that. I went on a 300 mi flight the other day and about 60 mi from destination my engine started to run a little rough, lost about 100 rpms. Picked an airport and ran through everything I could think of. Engine smoothed out-wanted to make it to an airport with mechanic on field so turned on course again, rough again. By this time I eliminated water in fuel and started blaming it on bad fuel. Smooth again then rough, then smooth. Landed and inquired for the mech and started making arrangements for draining fuel. He said I should try a mag check-back in, run up, great check 100 rpm drop each side. Called home mech, he said should have tried mag check when running rough and urged a check of plugs first then mags. Turned out two of the bottom plugs had cracked insulators. Four new plugs later I was on my way on an uneventful trip home. I hope everybody reading this doesn't have to learn things the hard way like me. '66E