Your description of the evolution for your panic threshold follows a very characteristic pattern for persons with the disorder, with sub-threshold symptoms predominating as a later course.

Benign palpitations have their basis in inappropriate stimulation of the parasympathetic nervous system, in particular a specificity associated with the Vagus nerve, or 10th cranial nerve, that innervates the heart, lungs, larnyx (which incidentally is why you feel a smothering feeling in your throat), GI tract, etc.

The symptoms you are experiencing are the consequence of this heightened activity due to stress or anxiety sufficient to stimuate the SNS/PNS. You should be aware that the palpitations are not arising from within the heart itself and does not represent any type of warning sign or prelude to a cardiac event of any type. As I've told many members to this forum, in over 40 years of practice I've never once even heard of a person undergoing a life-threatening cardiac event of any type as a consequence of the type of palpitations you and others to the forum are experiencing.

Changes in environmental or internal stressors, as well as slight changes or normal fluctuations in phsyiological status, can bring about remitting symptoms interspersed with periods of what you more know to be normal for you. Just as importantly, changes in bowel habits, ie constipation or other disturbance in the GI tract such as Irritable Bowel Syndrome, can increase the potential for benign palpitations to occur because the Vagus nerve is associated as the "gastric nerve" where it innervates the Gi tract. Signals sent via the Vagus nerve can cause the type of stimuation necessary for the palpitation events to occur.

Please read my postings entitled HEART PALPITATIONS 101 PARTS 1 through 6. This will give you an idea of how the body works in this regard to create the symptoms you are experiencing.

Although I have never experienced palpitations in a prominent fashion, it must be realized that these events occur within everyone because they are associated with an innate process known as "fight or flight" syndrome. The postings will help explain this a bit further. I will say that as a former combat pilot, I've definitely experienced my heart skipping a beat or two on a number of occasions upon sighting enemy aircraft and knowing I was potentially about to engage in aerial combat. It's the same process at work by the way, only in the case of anxiety there is no real threat, but rather merely a perceived one. Regardless, the body will respond as if the threat is real. This is why the symptoms are confusing, because folks can't understand why their body is reacting in such a fashion in the absence of any illness which would reasonably explain their symptoms.  

Please read the postings I described and we'll talk more.

Best regards and Good Health