I Have An Unexplained Increased Heart Rate That Runs 175

I have an unexplained increased heart rate that runs 175 and lasts for hours several times a month. I have been thoroughly by a cardiologist and was given a clean bill of health. I am 43 and yes I smoke and drink caffeine but I have decreased them both over the last year. However I am overweight and have had a thyroidectomy but even that is monitored on a regular basis and is within normal range. I have 1 grandfather that died of a heart attack at age 38, so this is very concerning to me that I am still experiecing this and with no explanation of why. It also always starts while I am just sitting around or even wakes me up from my sleep. Any suggestions?