Well, the first part of your post made me think immediately of Atrial fibrillation because my heart palpitations don't occur during activity either but rather afterwards when I am setting at 'puter, watching TV, eating, etc.
And some of your later symptoms have been reported w/A fib as well. But then others .... ???? The stress tests and echos, etc. can't detect A fib, not even the EKG, UNLESS the A fib is occurring whilst they are dong the test. If they don't happen every day a 24 hour Holter monitor isn't going to detect them either. For sporadic A fib you would probably need to go w/a 14, 21 or 30 day event monitor.
Other than that I have no ideas as you seem to have ruled out my other thought of a potassium, magnesium, calcium, etc. deficiency.
Good luck and God bless!
My computer says I need to upgrade my brain to be compatible with its new software.