What brand of wires and plugs? Sometimes low quality parts perform as bad or worse than the worn OE parts. Motorcraft plugs seem to give the best results, though others have replaced with good brands like Bosch and NGK. You might also just have a bad set of wires, or even just a single bad one.
It could definitely be any of the suggestions already listed, though if it only happens under load, my experience is it's usually the ignition or the fuel delivery.
My personal experience was in having hesitation with a loud knock under load, when hitting the gas hard it sounded like a sledge hammer was being pounded against the side of the engine block. Turned out the fuel system was dirty and the injectors/valves were being hampered from flowing the maximum amount of fuel/air from deposits caused by using gasoline with low detergent levels. After switching to using Top Tier gasoline (Chevron. Texaco, Shell, etc.) exclusively, the hesitation ceased after a period of weeks, and has not returned.
You might want to try a bottle of fuel injector cleaner in your next few tanks of gas to really start cleaning the system, but just sticking with Top Tier gas brands is usually enough, though it might take a bit longer to get everything clean on their own.
