What Causes Feeling Of Heart Skipping Beat With Hard Thump?

My heart feels like it stops (possibly skipping a beat) then thumps real hard (once) before returning to normal. Been to a cardiologist, had echo done, wore a halter monitor (twice), and have done a tread mill test. Used to happen once an hour or two, sometimes as little as once a day... now happening up to three to four times a minute. Happens more when I lay flat on my back, sitting up straight or lying on my left side. Also seems to happen more frequently after I eat, or exercise. Now beginning to get painful when it occurs. My age is 34, height is 5-10, weight 170. I actively go to the gym, take a multivitamin and fish oil. My diet is healthy (low sodium, lower saturated fat), and I seldom drink alcohol (maybe once a week) and don t smoke/use tobacco. What could be causing this? Thanks!