It's not the ignition timing that's jumping, it's the fuel sync timing.

I told my customer about the distributor drive gear and bushing, and how there's a TSB for it wearing too much. I also found a bunch of related forum threads throughout the internet with the same symptoms and they point to the bushing. I don't know if my customer is going to replace it, haven't heard back from him. I kinda want to do it just to see if it fixes the problem.

Thanks for all the help, guys.