Holley stumble trouble
Be a little more systematic.
Change Transfer slots .045 to .035 ( a big jump)
Tune 1: Adjust idle mix screws for best vac at 800 on the richer side (1/8 to 1/4 turn)
Tune 1 a: If idle is not at 800, write that down. Road trial or not.
Tune 1 b: Bleed in more air at idle. To avoid drilling the throttle plates for testing, crack the secondaries or put a restricted tee in the PCV line.
Test Reduced Primary IABs
Tune 2: Allow Transfer slots to be adjusted between .030 and .040 if needed.
Tune 2: Adjust idle mix screws for best vac or highest rpm on the richer side.
Trial that.
if starting from 1b, (bleeding more air in), decide whether to keep or trial without the additional air at idle.
Test effect of timing
Already know that large changes in timing do affect the tip in due to previous trial with and without vac advance.
Therefore measure the timing and figure the advances.
Test of 2 or 4 degrees less timing at idle is fine to learn the effect.
But know that it will may result in lower rpm, especially a 4* reduction. If in the previous tests, idle rpm came up (with overly opening the transfer slots), then timing can be used to get the rpm back down to 800 rpm or so.
If rpm did not ever come up, then if anything, just try 2* reduction and repeat previous tests (stuff done in tune 1 and 2) to see if a stronger idle can be made with less timing.
This would be tune3.
That's all very informative because now you will know the effects in idle and very low rpm.
But the issue encountered is in the 1500 to 2800 rpm range.
It may be related to too much mechanical advance between 1400 - 2800 rpm.
And it also may relate to too much or too soon contribution of vacuum advance.
Once the timing curve is plotted, it should be pretty obvious.
If you have an adjustable vacuum pump (mity vac or A/c pump) attach it to the vac advance and with the cap off observe the vacuum when the advance starts to move and when it stops moving.