I dont know what your running for initial timing but ya might try backing off the timing a bit like if your at 18* back it down to 16* this will bring down the idle rpm then you can add idle rpm with the idle screw on the linkage.
I’ve tried initial timing settings all the way from 10* to 22* and everywhere in between while recurving the distributor to achieve settings of 30* to 38* total.
The stumble seems to occur just as I start to give it a little more throttle during cruise. I have a vacuum gauge in the car and the stumble occurs at about 14” which is just about where steady cruise vacuum is -15”.
