Having the same problem with my car. Tried the Lotus chaps through the club but reply, below, was no help. Took the car in to SGT in Taplow where they replaced the coil. Got the car warmed up on the drive back from the dealer and gave it some stick and hit the 5500rpm limiter. I have accepted the fact that this is here for good. The Emerald ECU is looking like the only option other than switch off, switch on.
There was a recall campaign in July 2002 which required the engine ECM to be re-programmed to overcome a potential misfire at approx. 5,400 rpm. With the new programme, if a significant misfire is detected, The ECM will switch off the relevent pair of injectors and allow continued safe 2-cylinder operation without unburnt fuel entering the catalytic converter. A drop off in power would be experienced and a 5,500 rpm limit imposed by the ECM. Switching off the ignition for 10 seconds before restarting will restore 4-cylinder operation until a misfire is again detected. The cause of the misfire needs to be determined and rectified.