It could be a lack of body ground. You didn't mention it,so I don't know if you have one. There should be a ground from the battery to the engine and to the frame. I once replaced a factory cable that went from battery to body to engine with one that went battery to engine direct. I pretty much the same experience you are having.
Sometimes the electrical system will ground through a accelerator cable/speedo cable/etc. That may explain why it is intermittent. In my case, I caught it in the act as it arc across the motor mount where a junk cable was wrapped. That explained my intermittent start. A new ground to body fixed my problems.