Hello and thank you for using HCM. I carefully read your question and understand your concern. My opinion is that this floppy sensation on the heart has to do with some rhythm issues. There are some unusual electrical impulses that generates in different part of the heart and stimulate it. They cause an extra electrical stimulus that we call extra-sistolic beat, that s not a normal one. A person that experiences an extra systolic beat might feel a palpitation,a skipped beat, a sensation like the heart is racing or flopping in the chest. You also might feel difficulties in breathing deeply, anxiety ect. There are different causes that might generate this electrical stimulus of the heart, like simple emotional stress, coffee products, alcohol or different pathology like anemia, thyroid gland problems ect. So, if this flopping sensation happen just once in a while, it is normal, you have nothing to worry about. The fact that you have done the cardiac stress test and the holter monitor and they re normal is a very good thing. If the holter has not registered any significant arrhythmia, than we should not worry. So, if i was your treating doctor i will simply recommend you some blood analyze to exclude any anemia and your thyroid hormones to complete the full check up. If they result normal to, than we have nothing to worry. It might be just any random extra-sistolik beat that gives you this flopping sensation. You don t mention your age, but if you are able i will recommend some regular physical activity. Sometimes it resolves this rhythm issues and this sensation. Hope I was helpful. Wish you goo health. Best regards.