Flat spot & miss at low rpm

My bike has a terrible flat spot from ~2100 to ~2500rpm... it feels like you're about four gears from the wrong place. If you're in a low numbered gear (like 1st) it is a little flat and then lurches when it hits 2500 and rides 'normally' like you'd expect. If it's in a higher gear (eg. very low-throttle very low-rpm cruising like 5th at 60km/h on a flat road) then it's really flat and when it reaches the spot that it would 'clear out' in a lower gear it sometimes clears out but often just misfires - it almost sounds like it's forward-firing.

Rides fine everywhere else, on the dyno it's fine & the curve & afrs are much like I'd expect based on my last ZX12R even though this one is a little tired / lower numbers.

I've tried it with & without the power commander so it's not a mapping issue. Always run on premium.

The bike itself is a B1 ZX12R, I don't know much of the history but it's not exactly immaculate. It has a full system on it, otherwise it appears fairly standard. Not sure if there are any other unseen mods like air filter, ignition advancer, etc.

Any thoughts/opinions appreciated. Need to fix it for no dollars, or live with it

Have you tried running some injector cleaner through the tank?

i went to the stinger rally this weekend on my 12 and got to the sweepers going between the sugar cane paddocks it definatly has no flatspots or miss had to wait 5 minuets at the cattle grid for the busa to catch up i run injector cleaner in it every now and then

Pulled the error codes today.

Key on: FI light constantly on.
Bike idling: FI light constantly on.
Small RPM increase: FI light constantly flashing, no discernable break.
2500rpm+: FI light off.

None of these seem helpful as the book doesn't show anything for a solid light or constantly flashing light.

HELP!