Don't rule out hyperthyroidism. That's what caused mine, but caffine does trigger them. I could be sitting at my desk and all of a sudden my heart rate would jump to over 200 - very scarey! I was on medication for 4 months before my thyroid levels were back into the "normal" range. During that time I was told it was ok to exercise but to keep my heart rate below 160 to avoid stressing my already stressed out system. I've been off my meds for 10 months now - yeah!