I honestly dont know jerry, it can be at completeley random times very strange, My only other symptom is burning sensations on the skin arms, legs, shoulders, legs, again completely random make feel horrible for a small time like 5 mins then i feel completely normal again, sometimes i have a really hard heart beat but it is still beating in a normal pace and rhythem.
Those intermittent problems are the ones hardest to diagnose. The tests you have had and the echo should give a very through analysis of your heart's condition.
So, what else is going on? How about eating something that disagrees with your system, that's more what it sounds like me.