Under 'AFR closed loop' (in the pro tuning section. You typically need to go into the display settings and enable pro tune) you'll find:
AFR loop speed. I reduced this about by about 10 numbers.
Idle trim rate pos and neg, I cut these in half.
There's a section for 'idle stability spark' or similar. The default values are something like -3 -1 0 +.5 +3 +5 degs for various rpm fluctuations. I narrowed the range a bit and then also took a couple degrees out of my idle timing so that when it adds the max it doesn't cause a shudder. I've got a 408 with around 300 advertised duration, so my idle timing is 22-24 deg.
Last is the loop rate under 'idle control'. This controls the reaction speed of the IAC motor. I cut these values about in half as well.
I'd focus on the idle trim rate first, then the loop rate. Cut them both in half to start, and then maybe play with the idle stability spark if you still have issues. I'd leave this for last though, I think I wound up moving things less than a degree total from the stock settings.
