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there are a number of things which can cause this. The injectors or a faulty temp sensor would be most likely, or an issue with the glow system.
in order to verify and avoid costly guess work, you will need to replicate this issue at a time when an actual BMW specific diagnostic tool can be connected to the car and proper diagnostics carried out. RAC do not have the equipment for this so you will need to use a BMW specialist garage if you would like to pinpoint the issue prior to replacing parts. This approach is strongly suggested.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle.