Unfortunately the problems of the "blod" are not visible(or very rarely), so you need spare part and testing them one after the other there is no other solution. because the "blod" can come from the wifi card, a ribbon cable damaged, or one of the two screens (on the old 3ds the touch screen and the camera also can cause the problem and maybe can still be part of it). Almost all components are a suspect. So unfortunately no one can tell you precisely which component is the problem