Age 78, heart rate sometimes as low as 39 when arising in the morning,no noticeable symptoms. Also while hard exercising the pulse will suddenly drop from130-140 to 70-80 with no outward symptoms during the workout. After a few moments of continuing jogging, the heart will increase to 140 and slowly increase to a rate of approx. 155. Normal perspiration and uniform decrease in heart rate during cool down. My age is 77,good body mass index, cholesterol 175 with a good ratio, EKG and Holter and stress test show no problem, Please evaluate and advise.