My 90 750 seem to be running on only six cylinders. I just replaced the alternator because the alternator would not charge until after I reved it up to 2500 rpms. The new alternator has the same problem. The car is sluggish to get up to speed and shifts hard and MPG gage seems to be running half of what it is actually doing. I tested the Modules and they seem to be fine. Any suggestions

Thanks

Bill