1) Make sure you are breathing to the side and not lifting your head to breathe. Any lifting of the head will cause legs/hips to sink.
2) Make sure after you have speared that during the initial part of the underwater stroke the arm is not stroking downward. A downward stroke will lift the upper body resulting in lowering of hips/legs. Sometimes people do that without thinking to raise the head for a breath. All stroking should be directing water backwards for propulsion/anchoring and never downwards.