Are you taking plenty of minerals like calcium, magnesium, and potassium? I know that sounds silly, but I get extra beats when my minerals are off. I used to get SVTs and was diagnosed with Wolf Parkinson White syndrome and had a pretty "simple" procedure to correct it.
I don't know, but assume that hyperthyroidism could cause heart "abnormalities". If you are checked out by cardiologists and are fine, I would take that to the bank. It is scary to feel, but there are lots of heart issues that aren't life threatening and are just annoying.
My heart rate used to get to 230 for extended periods of time (15 minutes) and it was horrible! The worst that could happen though was I would pass out. Scary, yes and I did get it corrected. But all the cardios I saw were nonchalant because it wasn't life threatening.
Also remember, cause I've been there, that worrying about your heart may cause anxiety and make things worse. Try to relax and just let things happen. They are scary, but things seem normal. How about some yoga or a massage or meditation to help you deal with it....if it's going to be "normal" for you, you will need to find a way to cope with them.
Good luck mama.