When I got my m3 lump running the idle was not stable at all, I spent ages looking for vaccum leaks, and found none, finally I hooked up the BMW diagnostic gear, it pointed me in the direction of one the lambda sensors. On further analysis the signal cable on one of the sensors had rubbed through, this was giving sporadic readings when the engine was vibrating, this then caused the idle to hunt as it was receiving duff fuel info on one bank. May be worth checking? The cabling is easily fanged when installing the engine
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