My car loses power around 1500 - 2000 RPM, the RPM indicator goes up an down all the time it doesn´t stay stable while runing, is there some way to avoid this??

sounds like few problems there..vaccum lines if this car is new enough and you have a autozone there or advance they will plug it in machine 4 free..thats a start...good luck....

I'd chance a half - educated guess and say that you have a wiring problem... if your tach (RPM gauge) is bouncing, you may have a short wire somewhere.. meaning that you should check your ignition system, since I"m pretty sure it's no diff with BMW's, that the signal wire for the Tach comes from the ignition system.