It sounds to me like the springs are opening just barely too quick. That temporary flat spot might disappear altogether when the weather warms up. Another solution if you're willing to compromise on perfect F/A (but not so much on performance) is to over-richen the idle a little. You might have to change plugs more frequently, but a larger pilot jet would probably push a richer mixture into the transition area and solve the problem. It's easy enough to try.

'08 America Blue/White; Custom Headlamps, Custom Lowers, Clearview 20", Bafflectomy