Jo,
This is just food for thought- I had to start seeing a cardiologist last year and have been through the full range of cardiac tests because along with my high BP and cholesterol, I also have mitral valve prolapse. I started having problems with my heart racing and palpitating last year and bloodwork showed that my thyroid was a little off so my PCP put me on Synthroid and that seemed to help for awhile. When I went back to the cardio in Oct., to go through those tests again, I happened to mention to him that for a couple of months my heart had been racing and palpitating again, really bad, and wondered if I needed my dosage of Synthroid changed. He told me he didn't think that was the problem and wanted to try me on a low dosage of Atenolol, a beta blocker, and that has made a huge difference because the symptoms have pretty much stopped. I'm not saying that's what could've been going on with you but if others happen to ever have these symptoms and all the tests come back normal, they might want to have it investigated further.