During my initial OW lessons, which were just last month, I also had trouble staying down. Although I am only 100 pounds soaking wet, I had to increase my weight to 8 lbs in salt water with a 3m shorty. The strange thing that is different from your experience is that it was happening when I was trying to swim forward. When I was still and floating, my buoyancy was fine. When I began to start swimming again, I started to float upward. I spoke with the dive manager and her tip for me was to sing in the water. It immediately struck a key for me. Going back to my singing days, when you sing, you use your stomach muscles to help expel the air from your lungs in a more efficient, controlled manner. When I applied this concept to my diving, it worked really well. You might scare some fish away, but it might be worth a try.