Hi, I just saw this. You sound so similar to me. I have those issues as well, they scare the bejeebers out of me. I have discovered that the more I worry and think about them, the worse they become and come more often. I also don't drink enough water, dehydration can cause them too. I have a hiatal hernia, that definitely can be a catalyst for the skips from hell.
It looks like you had all the right tests and they were "normal" I go to the ER all the time and the test are "normal" too. I have to learn to believe and trust the doctor, I think that will help you as well. I know how scary they are, believe me, I have had mince since I was in my late 30's, I am now 66, so I am still here, they haven't killed me yet.
I hope that by now you are doing better and skip free.
Anyway, I have them more when I lie down as well, especially if I have had a snack or drank some water. Do you have a hiatal hernia, that can sometimes cause the extra-systols when falling asleep as the diaphragm is pressing against the cardiac reflex muscle. Also, the vagus nerve can cause this. Stomach pain can most assuredly cause heart skips as well. It might be a wise idea to check with your GP, he/she might refer you to cardiologist and recommend wearing a holter monitor for a day or a week. That way, you would know what is going on better.