I am 41 and having the same issues, I am fairly fit and training for my 10th half marathon. I am getting the same feeling where my heart misses a beat, and comes back with a real thud, again this is a real funny sensation
My doctor advised nothing is wrong and I can carry on with my training. I gave since cut done on the caffine and slowly going over to decaf, I have noticed the missing beat does not happen so often, so feeling more at rest that all is ok