Over 55 MPH causes bad mis-fire, Turn car off and re-start it runs fine.
What is the error code that you get on the engine? If you are having a misfire there can be several reasons. Clogged fuel injectors that can't keep up, bad spark plugs or coil packs, shorts in the coil pack leads, f'ed up valve timing or problems with the vanos (not likely), or carbon buildup restricting airflow into the cylinders. If you are a real DIYer you can replace these things first and grab a borescope off of amazon. Pop that down into the cylinders where the spark plugs used to be and if you see a ton of carbon, that's probably the issue.
Search MINI Cooper Questions
MINI Cooper Experts
Related Models For Sale