Intermittent rev loss

I have a 1990 820i with the M series engine, multipoint injection, and for some time it suddenly starts trying to cut out, the rev counter needle can be seen to fluctuate wildly. This can occur while driving along or when stationary, generally once the engine has warmed up and may carry on for for several minutes before returning to normal. I have the Fast Check diagnostic tests for the stepper motor, ignition and fuel ecu and they all show passes for the various sensors. I have recently fitted a new ignition switch and both the ignition and fuel ecu's so these can be ruled out. Does anyone have any idea what this might be caused by? It happens virtually every time I take the car out. Thanks, Paul

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