Yep, that low of vacuum at idle is most likely the problem. Mine idles at 17", and sits at 14" in drive. Vac advance doesn't drop out until 10".
Like supercuda says, I bet you're right on the edge of advance. As soon as you put some load on it, the advance goes away and your idle drops. This would be exacerbated when it's cold.
If you ever find yourself in a fair fight, your tactics suck.
