I have the same thing - it just started a few months ago. I was very concerned because I am almost 19 years out from a quad bypass. Doc identified the problem as PVC's (premature ventricular contractions.) Did a stress test and he said he was very pleased with the results. Put me on a beta blocker and episodes have been reduced but not completely stopped. I do not have chest pain - just that very uncomfortable feeling that my heart is flipping over. I do sometimes feel weak on standing - I think that is the med. He did say that stress and anxiety can contribute to the problem. Hope you are feeling much better soon.
Paulette