Can't keep idle

[447 LS2]

When backing out of a driveway & letting off gas car dips down 300 -400 rpms or comming to a stop.
I have Tired everything I don't want to drill a hole in the throttle plates
Help
Steve

[TAQuickness]

drilling or adjusting the set screw is only necessary if your IAC is near or at its operating limits. During hot idle, the IAC counts should be in the 60-120 range.

But what your describing sounds as if your throttle follower is decaying to quickly, desired idle speed too low, or that your base spark drops off too quickly

[Dirk Diggler]

Or not enough air in the throttle cracker when slowing back down and crossing thos MPH's