What Causes Low Pulse Rate Along With Dizziness And Fatigue?

My pulse is often 50-60, sometimes lower. I am frequently light headed and very fatigued. I exercise about 4 times a week and am very tired afterward. I fainted on a hike and nearly did again on a second hike not long ago. I sleep poorly often but not every night. I am border line angry and worried. My doctor has done the office electrocardiogram and twice sent me to a specialist for a treadmill test. He also did a full blood panel and my recent creatinine level recently was 1.54. My testosterone level was 368. My doctor doesn t seem to know what my problem is. What can you suggest?