Plugs might be clean but the resistor in them can fail or
conduct intermittently, weakening the spark. It won't usually show up
during a mag check, but in cruise, if the mags aren't timed exactly the
same, the plug that fires first causes a pressure rise in the cylinder
that the defective plug now has trouble sparking against. Plugs need to
be tested in a pressure-type spark tester, and even then they can act
up when hot.

Dan