Rapid heart rate was one of my worst lyme symptoms and one of the scariest. I did all kinds of tests too, but in the end it was just caused by the lyme and got better with treatment. It was just something I had to "deal" with along with my other symptoms. I was given beta blockers to slow it down, but I never took them because I hated being on so many meds at the time. My heart eventually went back to beating normal after long term treatment.
I hope they find something that will help you.