You've mentioned a couple of fainting episodes. Did these occur in relation to your low blood pressure? Have you ever been tested for Dysautonomia?
When I was hospitalized in april, a cardiologist did a complete work up. My resting BP stays below 100/70. I have fainted a couple of times and feel faint often. The doctors have said the word tachycardia. My pulse will jump around at any given time. It seems like when I get up and move around, I will feel better. Lying down is when I have the most problems. It is a very uncomfortable feeling. When I was in the hospital being monitored, I could feel something was not right. The replace battery light was lit up on my monitor. I called the nurse and she said it was fine. I know my body and I know when something is not right. How do I go about finding someone to help me?
What is your normal resting heart rate? If it is over 100 bpm, you are in a problem area and should be on a medication to lower your HR. That said, most (all?) beta blockers used to lower the HR also lower the blood pressure, you don't seem to need that. I'm surprised you didn't pass-out with a BP of 70/50.. well maybe you were sitting down at the time and didn't need a lot of oxygen.
Did any of the doctors say "tachycardia"? Is your heart rhythm regular, no jumping around? You didn't mention a cardiologist, have you talked with one about your heart problems?