I will provide an honest answer. I trust this will be appreciated. Please remember that a low rating PUNISHES me for being honest and that I receive nothing for my time until a positive rating is clicked.
without knowing if the transmission is shifting, it is difficult to say what this is
if the transmission is not shifting and is sticking in low gear, then this can be due to low transmission fluid, a faulty transmission temp sensor, or other internal transmission failure.
if the transmission is shifting, then the likely cause is a failed turbo.
I would strongly advise that you invest in a proper diagnostic at a main dealer or BMW specialist that has the right computer equipment to plug in and monitor all inputs and outputs from the engine and transmission management systems - this will resolve the uncertainty for you and allow you to have the correct repairs made without undue guess work.
Sorry if you were hoping for a different answer, please don't shoot the messenger - I can only give the honest answer, which I have. Please remember that a low rating PUNISHES me for my honest and prompt response, and I receive nothing for my time and answer until a positive rating is clicked.
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.