I think the major points have been covered -- if your suit is auto-venting, either you are significantly out of trim, or you are overweighted. It should not be difficult to retain the gas to balance a single tank's worth of air in your suit.
But in addition to static weighting, which can be a problem, there is also body posture. If you tend to flex your hip joints and bicycle kick, it will tend to drop your legs. In addition, if you aren't bending your neck, you are probably rearing your body up to be able to see in front of you. You can play with these ideas with a buddy, just by adjusting your posture and asking your buddy to give you feedback on your trim, but it does help if you have someone in the water with you who knows what the likely problems are. I think if you do a formal weight check and fix your posture, your trim issues may resolve themselves.
But in addition to static weighting, which can be a problem, there is also body posture. If you tend to flex your hip joints and bicycle kick, it will tend to drop your legs. In addition, if you aren't bending your neck, you are probably rearing your body up to be able to see in front of you. You can play with these ideas with a buddy, just by adjusting your posture and asking your buddy to give you feedback on your trim, but it does help if you have someone in the water with you who knows what the likely problems are. I think if you do a formal weight check and fix your posture, your trim issues may resolve themselves.