this is common on RB25DET
coils usually have small hairline cracks, so spark arks on head and is weaker,
ont high boost, ECU runs rich, and retards timing to protect engine. At this point the cracks in your coil become noticable as your car starts to hesitate. The problem is most apparent at 4500 -> 6000 rpm
cheapest solution is:
araldite, silicone or whatever the cracks on the coils AND get a SAFC to lean out your mixtures.
gapping down your spark plugs may make it less noticable, but won't fix it.
best solution:
new spitfire coils + new stand alone ECU
also the more expensive solution
I have gone for the cheapest solution, and my car runs incredibly smooth. The problem does not exist anymore
Read through this thread, as it details where to cover the cracks on your coils.
Invest in a SAFC to adjust your Air/Fuel ratios - you will NOT regret it.