Coast-down lean roughness

With a 236/236 cam in my 396LT4 I've got most aspects of driveabilty sorted out except for one. I'm using a FAST computer and when I'm descending a very light grade I get a bit of rougher coast-down than in steady-state cruise.

I also notice my A/F gauge go lean when this happens, even though the target in these cells is still 12.5:1

It's almost like the ECM thinks I'm idling or going into pure decel, but I've set my max TPS for idle just 2% over no throttle at all.

Any suggestions on how to improve this aspect?

Thanks!

do you have a Decel timing table? What are you iac counts? might want to close the iac further more for decel, & maybe shut off the injectors all together while controling the decel rate with the iac. Not sure how it works with FAST though.