Gm847 355 ported heads idle.

[anniversaryss]

I'm have a 355 lt1 with gm847 cam and ported heads with other goodies.
I got my second tune back from my second tuner and my car will idle fine in park and in gear with some fluctuation.
It's only when i drive and come to a full stop does the idle dips down to about 500 catches then back up. Sometimes it stalls.

From what I know this could be bc of iac counts.
Being a 97 I don't have any software to log it but I would like to make this better.
I know drilling the IAC hole bigger may improve it or cracking the throttle blades and adjusting the TPS will help to.
What method would be the best without a scanner?
And if I do the IAC port how big a bit should I use?