I had a similar condition years ago called Paroxsismal arythmic tachacardia (spelling is likely wrong). I would wake up in the morning to find my heart absolutely racing. The first time it happened I drove myself to the ER (which I then found out was a NONO) and my heart was beating 198 beats /minute. on one occasion it was beating so hard it actually had my PJ's flapping.
Anyway it has something to do with the valves of the heart.
It was corrected by some very minor (laser
) surgery...in and out the same day. You might want to ask your doctor to check out your heart valves.