Whens the last time you changed the transmission fluid? filter inside it might be clogging up if it has been a good while. as for the bogging, is the cel light not coming on? surely if it was the plugs the cel would pop up showing misfire

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1999 328ic M-TECH
"only 2 cars in my life have every survived my prolonged lead foot.. a Nissan Maxima and my BMW.."
BEHOLD THE POWER OF 3!!