Palpitations are often stress related. I used to have sporadic one when I was really stressed out, but they end up totally disappearing. You probably know but as an actionable first step you could try to reduce your coffee consumption and see if you are getting better after a week.(assuming you are drinking coffee)
Taking some magnesium supplement could also reduce the symptoms, since a low magnesium level, caused by stress, can also lead to arrhythmia. Typically every time I am low on magnesium I experience eye twitches that are quickly disappearing after a few week of magnesium supplements. I am surprised it was not prescribed by your GP or cardiologist.
I don't really want to give medical advice carelessly but since you have seen a cardiologist the most serious cause have probably been taken into consideration.
http://www.health.harvard.edu/fhg/updates/Surprise-of-palpit...
Taking some magnesium supplement could also reduce the symptoms, since a low magnesium level, caused by stress, can also lead to arrhythmia. Typically every time I am low on magnesium I experience eye twitches that are quickly disappearing after a few week of magnesium supplements. I am surprised it was not prescribed by your GP or cardiologist.
I don't really want to give medical advice carelessly but since you have seen a cardiologist the most serious cause have probably been taken into consideration.