Hello,
well you can check your coil at a local garage, as well as the controller (regulator?) of the alternator.
So is it an alternator, or a dynamo ?
Guess the latter ...
Usually here in Europe Bosch parts are a lot cheaper than BMW ones, but they are just the same - only the "BMW part" has a BMW logo imprinted on the Bosch part lol.
But i would first check the filters, and the fuel pump ... losing performance at higher revs, or load, is usually clogged/damaged fuel system or coil.
It could also be gas bubbles in the fuel lines, if there's a damaged heat shield.
I once had a french car, phantastic in winter and design, but with a german Bosch electronic ignition and fuel injection system. All the people always talked about bad french cars, but it was this Bosch ignition system that made it that unreliable. Since then i write it as "Boche", like swine
.
It took me 3 months to find out what it was (black box ignition system), and the local Bosch dealer had not found it - but it's understood he took money for searching 4+ hours, 300 Euros or so.
Greetings,
Catfish