had a similar problem with a mates escort coz,car was missfireing around 3500 to 4000 rpm which according to "stu" was a sensor failure and was particular symptom with p8 ECU's (4000 rpm limit is introduced for sensor failure) the car would run ok after that rpm and up to to it.
turns out we had changed a turbo a few weeks previous and the lambda sensor was duff so problem solved with a new sensor.
but your running a L8 and i dont think it gives this limit for a similar failure so i,m not sure but if you have changed all else as you have listed it might be worth trying the lambda sensor if its fitted