Stumbling under high load and high RPM

I'm new to the Jimny platform but am a die hard wrench head mostly familiar with old land cruisers.  Picked up a 2004 Jimny JB23w with the turbo 660.  It has roughly 80,000km on the clock and was in import from Japan.  Been a pretty nice little ride.  I had a coil go out a week or two ago and I replaced it and it was running swell again.  Now its started losing power at high load and high rpm.  Like WOT it stumbles and cuts out but doesn't stall.  Felt to me like a lack of fuel or something but with no servicable filter I'm not sure where to look?  At lower power outputs it runs perfect.  It will still cruise flat highway at 100kph but when you lean into throttle for the last bit of jam it stumbled and power drops right out...... easing back out of throttle and it return to running smoothly.  

Where should I look?  What to check and confirm?

I have a new set up plugs and 2 more coils on route to replace but don't think this is ignition related and maybe fuel or boost related?

Thanks,
 

Unfortunately we don't get the 660cc turbo engine here so there isn't much experience with it.

It is possible it is a spark, you need a stronger spark at higher revs, especially at higher boost levels to ignite the fuel/air mixture and if it only started after the faulty coil that does point in this direction.

However, I think you might be on the right track with fuel, can you get fuel pressure tested or perhaps the injectors?

Another general issue is the turbo, it might be going over-boost, possibly caused by a fault wastegate. If the ECU detects over-boost it will kill the fuel, drop the revs down and then cut back in, this would be your 'stumble'.