I keep having Blackouts or fainting spells

For the past 3 years, I have had symptoms of what feels like dizziness, nauseous, fast heart beats, then I black out. This has happened 3 times now, each time, I go and test are ran on my heart, CT scan, blood test and nothing is found. I am 62, no history of any diseases, high blood pressure occasionally, or sporadically, thyroid removed, high cholesterol. Any thoughts on why this keeps