I just checked timing on my 440/500. I have an MSD distributor set to bring in all timing by 3,000 rpm, which it does. But if I keep accelerating beyond 4K or so, the timing starts retarding, ultimately by about 7-10 degrees. The timing is steady, not erratic. It advances steadily, then retards steadily with increasing RPM.

I have a collar on the distributor shaft to keep the distributor drive from "walking up". It's a solid roller cam, and I have a bronze gear that's showing some wear. But I can't see how this might solve the problem.

I think I'll now pull out the distributor and try to replicate the issue on the bench with a drill, but thought someone might have seen a similar problem before