I have an RV-6 with an IO-360 EXP, dual P Mags, using NGK auto plugs. My engine randomly runs rough at full power, just after liftoff on three occasions, I have observed the #3 cylinder EGT drop to below 700 degs while the other three cylinders are 1200-1300 degs. Reducing power resolved the roughness in each case and the #3 EGT went back to the 1200-1300 deg range. After today's event I landed and did a high (not full) power run-up at 2400 RPM. I checked each P MAG individually and found no problem. I did one trip around the pattern with no roughness. When this happened a few months ago I installed new plugs in my #3 cylinder on my A/P's advice. The problem returned today. This problem is persistent but random. Has anybody else encountered this circumstance? Any suggestions?