irregular frequent thumps in chest

For over a month I have been having a strange intermittent thumping in the left of my chest, especially when I am sitting, (straight and also reclining).  I do not have them when I am active (tennis, walking, biking, housework, etc.)  I sometimes have them while lying quietly in bed.  There is no pain.  The thumps are irregular, 3 to 12 seconds apart.  They begin and end with no apparent cause. They sometimes last up to an hour or more. The thumps are so strong that they are easily felt externally, sometimes even visible.  They feel a lot like a baby kicking during late pregnancy.

I went to ER when they first appeared, as I was anxious.  EKG and other tests were all normal. Dr. had no explanation.

I am a 70 year old female, overweight, with high cholesterol.  Otherwise in good health with lots of energy. Very active.