Pounding heartbeat/heart palpitations.

I have had heart palpitations episode 6 times in last 6 months, 5 days before I woke up with rapid heart of 159 and I went to do ECG, the report said Synus tachycardia with HR of 139. I consulted cardiologist and he DID ECHO 2d test and my report was normal and he prescribed TAB cardarone 100mg and Petril Bata 10 for 1 month. 2 days before I again woke up with rapid heartbeat it was 125 BPM and I went to do ECG and it was normal. I have done my thyroid profile test and it is also normal. Whenever I am sleeping I can listen my heart pounding loudly that I can count my pulse easily. If I sleep on my chest that the heartbeat starts increasing. It makes me very uncomfortable. Pls suggest what needs to be done in this case. Should I continue with medicine or go for another test.