I bought a second hand mini cooper 04 a few months ago. Not long after buying it the engine check light came on. It stays on constantly, and flashes on and off usually when you hit about 50-60mph. As soon as the light flashes, it feels like the car 'chokes'. Looses power and jerks when you drop the speed. The coil pack, spark plugs and leads have been replaced as well as the O2 sensor. No one seems to know why. Any ideas?