I looked at the forum on the link provided but didn't see anything about why the timing should be erratic at idle with the 123. I didn't look through all the posts, so I may have missed it.

On the two that I've installed, each had jumpy timing when the timing light was used, and this was true both at idle as well as higher rpms. Sounds like these two were different than the "normal jumpy at idle" you're referring to. Nothing in the provided instructions said anything other than adjusting the timing with a stroboscope - that is, no mention of jumpy timing marks at idle. It's hard to believe that this is intentional or acceptable, especially given that there's no mention of it in the installation instructions.

I'd love to fit my 122 with a programmable distributor, or at least one that offers a variety of curves from which to choose. I've thought a lot about one of these, but haven't yet convinced myself to take the gamble. It'll be really hard for me to spend money on a device that gives erratic timing given that that same device is intended to cure... well... erratic timing. Seems more than a little odd.

How is the jumping timing light supposed to be beneficial?

Best,

Cameron
Rose City