A few questions to help pin down your problem.

I assume when you say it “turns” you are referring to the helicopter’s nose changing heading but the rest of the helicopter stays level. Depending on if you have heading hold turned on or not this may be normal, as long as it is a slow drifting motion and not a sudden spin. Vibration to the gyro can cause this effect to be magnified. The Gyro has 2 functions. In normal mode it is only used to stop the rotation. In heading hold the gyro will not only stop the rotation, but it will attempt to return the nose to the heading it was on before the drift or change occurred. A gyro can be fooled and this will cause gyro drift. In a hover, you will notice it a lot more than if you are in forward flight.

As for the engine. When you say it keeps going up and down, do you mean the RPM of the engine is rapidly changing up and down, like the engine is surging? Or do you mean when you start with a full tank the engine tends to speed up as you use fuel? As for a starting point for the low speed needle, that’s a hard one. Each engine is different. Your setting will change with humidity, temperature, and barometric pressure. Do you have a head speed governor set up. If so, the governor may not be set up properly.

I like to run my engine real rich. I get less power out of it, but it will make the engine last longer and I don’t do any 3D flying so I don’t need the power.