Hi, I Have Some Concerns About My Heart Rate. I

Hi, I have some concerns about my heart rate. I am an avid mountain bike rider/racer. Male 46 years old 200lbs. 5’8”. When I ride, or run for example 1 hour my heart rate gradually moves up quickly within the first few minutes to about 125 bpm then at some point usually 20 minutes in my heart rate shoots straight up to 180 to 200 bpm and stays up there until exercise ceases. It is not uncommon for me to have an average heart rate of 180 bpm for periods of more than 1-3 hrs. After my heart rate shoots up high I seem to have issues with balance and weakness in my arms and neck.