Another vote for magnesium here. It's a difficult mineral to get in the quantities you need unless you eat a lot of foods that tend to contain it like certain types of
nuts and other veggies.
The supplement I take for that is Twinlab's ZMA Fuel. It also has zinc which similarly is difficult to get in decent quantities (necessary for proper immune function). It's also the chelated variety for both which is absorbed much more efficiently than the metallic salts like oxides or sulfates.
I used to have a co-worker who was having an irregular heart beat. I gave her some chelated magnesium and the problem cleared up in a couple of hours.
Still, even if that works, you need to get a complete cardiac workup done - carotid artery scan (which will show the general state of your vascular system), echocardiogram which will show things like ejection fraction and any hypertrophy or dysfunction, cardiac stress test and a few other tests. When I had mine done about 6 months ago, it took several visits but it's worth it know what's up and what you need to keep an eye on.