Just a thought, but have you looked at dhydration and electrolytes? It's quite a basic really, so you may have already, but low electroclytes can cause palpitations and racing heart beats. I had quite bad palpitations for a while this year (they were even waking me up at night) until I looked into electrolytes (I think it happened because I had changed my diet quite drastically). The result was that I added 2 glasses of coconut water and a couple of bananas per day to my diet, to up my levels (mainly potassium - we need about 4700mg per day and most don't reach that). My palpitations stopped within 24 hours and if notice anything with my heart now, electrolytes/dehydration are the first things I look at.
Magnesium is also important - again most are slightly deficient and it can affect the heart like all electrolytes. I now soak my feet in a bucket with warm water and 500g epsom salt for 30 mins 2 or 3 times per week (you could also soak in a bath but I just find the foot soak easier).