I don't have much experience diving in a 3mm, but 12-14 lbs. doesn't sound like a lot to me. Am I missing something?
Anyway, if you sink when you're head-down but not when you're head-up, I guess you could be over-weighted, and just be unconsciously finning to compensate, which works in every other orientation. Have you ever tried going head-down (or head-up) high enough in the water column to have some margin for error, and seeing if you come back up when you inhale? I haven't tested this out, but it makes sense that if horizontal trim creates more drag moving up and down, then vertical trim would make you float higher and sink farther with each breath.
Anyway, if you sink when you're head-down but not when you're head-up, I guess you could be over-weighted, and just be unconsciously finning to compensate, which works in every other orientation. Have you ever tried going head-down (or head-up) high enough in the water column to have some margin for error, and seeing if you come back up when you inhale? I haven't tested this out, but it makes sense that if horizontal trim creates more drag moving up and down, then vertical trim would make you float higher and sink farther with each breath.