What Causes Heart Missing Beats, Dizziness And Lack Of Concentration?

I am 52 and 5'2 and 10 stone stilghtly over weight, with little exercise, I have always had low blood pressure and heathly. I have hd missing heart beats for the last 5 years but they have got worse, I can cope with them but the dizzyness and lack of concentration they give me in concerning. In the morning I am woken with a strange dizzy feeling in my head it feels like it will explode and I cannot think straight, I check my pulse and miss heart beats, every 3 or 40 the rate number changes, then I feel okay for a few minutes and this carries on for about an hour making me tried and weary. I have had a stress test and heart monitor for 24 hours and have been told I have a strong heart but still worry about this condition. It mainly happens when I am asleep as it wakens me up, but can happen during a meeting, it is not the meeting that is stressing me it is the missing beat that concerns me as I cannot think straight.