Heart fluttering / Hard Beat

I am a 30 year old, slim healthy male and for a few years have been experiencing heart "palpitations" which are experienced as a fluttering/quivering sensation under my chestbone that lasts for anything from half a second to 10 seconds. The events have been very painful at times when they happen even tho each incident lasts for such a short time. My doctor did lots of tests - an ECG and heart monitor (and an MRI as my family has history of right-ventricular dysplasia) and found nothing. He put me on atanalol which seemed to help at the time (I wasn't having any problems the week I wore the heart monitor - typical!)

They went away for about a year, I stopped taking the atenalol and I didn't notice anything and thought they had gone. However over the last couple of weeks they have returned and are being experienced like a fluttering/quivering sensation under the breastbone in the centre, followed about a second later by a deep (lower down in the chest) hard/forecful heart beat.

Any ideas what may be happening? I have read up about the atrium (top of the heart) and ventrical (lower in the heart), and it does seem that something is happening higher up in my heart (the quivering sensation), followed by this hard/thud of a heartbeat just after, before the heart goes back to beating normally. The last week I have been experiencing this (sometimes very badly, sometimes more subtley) about once every 20 or so minutes.