`10 FE390 spluttering at around 1/4 throttle

I've recently acquired a `10 FE390, EU version, and just had a few rides on it, and apart from a problem it's a great bike (I also have a fun XR400 - high-comp 426cc, USD forks, juice clutch, etc.).

The engine hesitates, splutters around 1/4 throttle, also when stationary in neutral racing the engine holding constant low RPMs, and also under acceleration. It starts and idles great, also pulls strong and clean when not near the 1/4 throttle. It never stalls, also pulls well in high gear from low rpms.

I think this problem can't be because of the fuel pump or filters clogged. Problem is the same with the stock and FMF 4.1 can, so I think it's not the mapping being lean, there is no popping at deceleration.

I suspect the spark plug, injector, or maybe some sensor (TPS?), I also haven't checked valves yet. I will change the plug, and try to clean the injector, but please help and advise what else to check. Do you also think based on these symptoms can I rule out the fuel pump and filters?

I'm running the ignition in "aggressive" mode, with a 6K8 resistor plug, haven't yet tried it in "stock" mode.

Many thanks in advance.

Thanks for the suggestions, Noppy. No blinked EFI error codes, no chaffed wires near the shock. I think (hope) this will as simple as a failed spark plug, the hesitation/sputtering sounds very much like that. Unfortunately I can get a new plug only in a few days time.

Got a new spark plug, it SEEMS the problem is solved, racing the engine in neutral, holding constant throttles works perfect - but a ride will prove it.

The plug I took out was LKAR8A-9, so it's not the OEM Iridium type (LKAR8AI-9). The hexagon part of the plug was in a bad shape, it definitely doesn't look like a 3 year old plug.

I hope it was this simple.