I have this issue going on with my ignition. It's been going on for a few years, and I haven't been able to figure it out. Now I'm at it again. It's really making me crazy. This is a Duraspark 2 system. When I point a timing light at the balancer when it's at idle, the timing marks randomly bounce all over the place. It's intermittent. Sometimes it's stable or it'll waver up and down a few degrees, but most of the time it's random. Regardless of what it's doing, the idle is for the most part smooth. Sometimes it sounds like it has a few misses. As I give it some rpm, the marks will stabilize more and more as it advances. By the time the timing is all in, it's solidly in one place. I've replaced almost everything in the ignition with no changes. So far I've replaced the reluctor, pickup coil, mechanical advance plate, vacuum advance canister, cap, rotor, plug wires, spark plugs, coil, and the ballast resistor. I have two ignition modules that I've tried swapping out. Most recently I replaced the wiring with a Painless wiring harness. I had a couple spade connectors going to the ballast resistor that I thought could be a problem. At one point, I pressed the connectors firmly together, and timing readings stabilized. When I let go, the readings went back to being random. The other day I soldered these together and it made no difference. I just dug into my distributor, and the pickup coil makes me feel suspicious. I see there's fine metal shavings stuck to the magnet, and I found that the top of the pickup has been rubbing against the reluctor. If I push down on the pickup, it separates from the reluctor and the whole plate rock side to side a bit. I measured the space between the reluctor and pickup at 0.015", but the magnet pulls itself into the reluctor when I let it go. I doubt it's suppose to do this. In a distributor disassembly video someone had two rings that clipped around the shaft above the pickup plate, and I'm missing one of them. Maybe the missing ring is supposed to hold it down more securely? I'm not sure if this issue would result in this behavior. I previously installed a pickup that was spaced too far from the reluctor and the engine barely functioned at all. Looking around I found that there was a pickup made that had an adjustable gap, but I have no idea where I could find one. I'm not finding many places that sell these pickups. Any ideas?
You didn't say, you are disconnecting vacuum advance at idle? I'm not sure about Duraspark but points ign has a rubber sleeve on mechanical advance limit pin (under plate with weights). Common for it to deteriorate allowing advance to wonder around at idle.
