On takeoff in my RV-6 I experienced rough power at full throttle. Simultaneously, I noted a significant EGT drop on #3 cylinder to 600-700 deg range versus 1300-1400 deg on the other three cylinders. Rough power ceased with reduced throttle. Conferred with my AP who suggested a bad sparkplug or two. I swapped out all NGK BR8ES plugs for same type new plugs. No problems in five hours of flight time since change. My RV has P-MAGS on an IO-360EXP. Replaced plugs had 80 hours of use. Replaced plugs all appeared to be in good shape.