I have had premature ventricular contractions otherwise known as ectopic heartbeats since I was in my early 20's and I am now in my late 40's.
I too have had the echo and loads of 24hr ecg's usually have one every 3-4 years when my nerve breaks and I am worrying about the ectopics and beg my Gp for another holter. I have been told over and over that these beats are unpleasant ( yeh!) but totally harmless.
Mine always show up on holter as I have them every day - on good days I only notice them a few times - at bad times which can last weeks it is every 3rd heartbeat!
We all have a harmless arrythmia and once they have excluded any heart disease which they can do with the echo and the holter then you can be totally reassured that your ectopics are anxiety related or caused by excess caffeine etc When I say excess some people are so sensitive to ti they can't have any.
Acid reflux can bring them on as they are caused by the vagus nerve being irritated and this lies accross the top of your stomach so a big meal or acid can cause them as can low blood sugar.