I have this, too

I'm at the opposite end of the Fach scale from you, and I have this issue as well. I've only just started to figure out how to fix it for real, and it's less complicated than I thought it was! Here's the thing: breath pressure works differently in different parts of the voice. The breath you use at the top is more tightly focused, and when you go back down into the middle, especially the lower middle, the trick is to cut back on the pressure without reducing your support (and core!). You just can't use the same amount of air (or at least air pressure) down there, especially if you just came from way up there. I also think of it in terms of finding "little" vowels - the most closed positions possible for my open vowels, less space in the mouth, abdominal support absolutely solid. I don't know if this helps at all, but I hope it does!