Here is a thread that contains a few possible answers.

https://board.moparts.org/ubbthread...07/re-idle-tuning-issue.html#Post2924307

Cab I will try backing down the timing a little.
It is a MSD distributor. I haven't checked the timing in gear I will do that. not sure what the initial timing is i put the strongest spring in the dist to limit the advance. Fuel pressure is set at 6.5 psi.
I use a MSD timing light

Try using the weakest springs you can get in there
I use the Mr. Gasket 925B spring kits in most of the Mopar distributors I use in Hi Po street and race only cars, I shorten up the advance slots so I have between 14 to 18 degrees at a low idle RPM and all the timing in before 2000 RPM. I really concentrate on having no more than 34 to 36 degrees total timing, higher altitude use 36, lower, below 2000 FT above sea level, I use 34 total
I also verify my timing tab and balancers when assembling the motors to make sure I'm getting what I see with the timing light and timing marks