base timing keeps advancing by itself...

Yup, a slipped balancer wouldn't effect the idle speed. Sounds to me like the point plate may be moving around. Any play in it or the Pertronix pieces? The only other thing I can think of would be the end play on the camshaft is way off, or, it's thrust surface and/or plate is badly worn or missing. What does the timing look like with the light? Is it steady and smooth when it's advancing, and returns the same way?