I have had heart palpitations for about 8 years, they just started happening randomly and realy panicked me! For me it feels like my heart pauses for a second and then a sudden hard thump. Sometimes it feels like it suddenly twiches/beats fast for a few seconds and I have to move and walk around and it goes. When i first had them and didn't know what they were it got them everyday, but on and off. I had a lot of tests, I had a 24hr heart monitor attached to me for a week - all tests came back fine. I have a lot of ECG's and a heart scan and all were good. I still get them, but maybe once every month for about a week on and off, sometimes can go a lot longer without them. I know how hard it is to ignore them, but the more you think about them the more they happen! If all your tests have come back okay, when you get one, just tell yourself it's perfectly normal and to stay calm. Coming from me being the biggest worrier of all time, I know how hard it is! But 8 years later they don't bother me anymore! Sometimes it makes me jump when they happen! But go back if you are not happy and want more tests!
Btw i have noticed that i get them around my period, so can be due to hormones. I also sometimes get them from excerise which wtill worries me when it happens! And also if I eat too much!