OK, me too, I am a very healthy and active 50 year old man that has had chest "flip-flops" off and on since I was 20 years old. I thought it was heart related since is was right near my heart (just under). Once in a while it will flip flop for about 5 minutes!! really disturbing. I had a heart monitor attached when I was 20 but nothing showed up. Last week I had it attached again since the flip-flops were increasing - I recorded at least 20 of these events but the results came back that I had a very strong and regular heart. So something else is going on. It feels just below my heart. I will try the magnesium suggestion.
TJ