You may have what's called gastrocardiac syndrome. Some people find relief by losing weight, eating smaller meals, and taking a magnesium supplement.
Believe it or not, fasting (e.g. skipping dinner if you experience symptoms either at night or in the morning) may also help.
You described me to a T, although I am not a vegen. It is very wierd that standing immediately re-establishes a good rhythm. This may not be life threatening, but sure is difficult to live with. Had a bout 2-day.....I couldn't wait to get out of the car. As soon as I stepped out of the car my heart resumed it's normal rhythm. To put it bluntly this syndrome *****.
Marya T.