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Plugs can still spark but miss under a load. If the bike has done a lot of idle time put in a fresh set or soda blast the ones you have to remove ALL the deposits on them.
Or the spark could be significantly underpowered. Check resistances for the coils. It could be a fault in the plug caps, the wires (although you've said those have been replaced), or the coils.
Also, if the wire-to-plug-cap connection isn't right it can result in extremely varied performance.