M roadster idle fault

Hi everyone, I've got a 2001 (3246cc, 325bhp) M roadster. When changing gear or approaching a junction when the clutch is depressed the engine rpm sometimes drops below 500 then returns back up to the normal idle speed. Also when accelerating it can very occasionaly have a flat spot not too dissimilar to a misfire. I have taken it to my local BMW specialist who has had it on the GT1 diagnostic system four times. Each time it registered no fault on any electronic component. They did suggest it could be the throttle position sensor, air flow mass meter or a throttle return valve as these were servicable items on earlier M powered cars. They also told me that an electronic component fault would have to stay on for about 10s for it to register in the ECU memory so even though it comes back OK on their machine it could be anything. Has anyone else had a similar fault or has any other ideas to what it could be? Many thanks Paul