Point in frustum test looks ok, triangle in frustum is definitely wrong, triangles vertices might be outside of frustum, but part of the triangle still inside it. From the pictures I guess you change perspective matrix after extracting the frustum.
Not related to problem:
Don?¢_Tt try to cull all triangles, you?¢_Tre video drivers most likely do that for you it makes more sense to cull large polygon groups with their bounding boxes or any king of partitions. And immediate mode is evil.