base timing keeps advancing by itself...

@Phreakish your explanation was perfectly clear and easy to understand. And it turns out you kinda nailed it. What the op said Very early on must not have been true at all about turning the distributor back further than his marks. As we now know what his issue was.

I'm sure it was true, the timing probably needed to retard further than the original idle timing to get back to a normal idle speed. If I was fixated on the timing (it's happened before), I can imagine thinking it needed to be that way.