According to my manual, the CG should be between 3&3/8 inches and 4&1/8 inches back from the leading edge of the wing at the fuselage sides. You should check it inverted (plane upside down). In the ARF version there are two ways to mount the servos. In a servo tray using control rods if your engine is light, or in the tail if your engine is heavy. If your .71 is not heavy and your servo's are in the tail, that might be your problem. I'm just a dumb 'ole country boy, can't convert inches to metric.

Hope it helps