RE: Could you give me an adivice about Traxxas Rstler VXL, please?
#1 is common with sensorless BL systems. It's called cogging. That is the rotor passing through each magnetic field in the motor.
#2 is tire balance, stick on weights will cure this. Nothing sounds "wrong" with the truck. Balancing the tires is something you want to do.
-JB