Here's my assumption (though, I only have 75 dives under my belt so I could be wrong--we didn't delve into in class as to why it was driving to make me dive negative):
Keep in mind, it isn't this trail of bubbles leaking from my nose right now that is the issue. By inhaling through my mouth, I'm bringing a certain volume of gas into my lungs. By exhaling through my nose, I'm expelling a different certain volume of gas from my lungs. Typically, because the passage is smaller through the nose, I believe I'm expelling less gas when I exhale than when I inhale. This, over time, inherently makes me more buoyant.
So, I dive with less gas in my wing (negative) ... until I do exhale all the gas from my lungs and then I start to sink. So, I add some gas to my wing, readjust my depth, and continue. But the cycle starts again, and this time I start to rise ... so I dump gas and am negative again.
At least, that's the best explanation I can think of.
What I know for certain is this: During Fundies, I was having a terrible time controlling my buoyancy. When we discovered this, I began making every dive with my nose plugged (with my fingers) until I could train myself to breathe only through my mouth. In doing so, I had my buoyancy in check to the foot. It was an amazing thing.
... of course, it could all be psychosomatic, so who knows.
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In the meantime, I will try to use a nose clip, switch off the Venturi assist, and attempt to replicate the issue without the mask to see if there's a more obvious way to pinpoint the issue.