v-6 hesitation

2000 f-150 v-6 5speed, 68000 miles. Changed fuel/air filters,plug wires, plugs. On occasion it runs terrible, hesitates, misses, bogs down. Ford dealer finds nothing, suggested a carbon cleaning which I had done and no change. Runs good for a couple days and then "acts up". No codes ever. Very frustating, any suggestions would be appreciated!! Thanks!