This is common with O-320 carburetor engines when in the leaned
condition... Going full rich for three seconds before changing the
throttle normally smooths this out... A possible cause is worn shaft
bushings where you get air leakage past the butterfly shaft with
movement... Another possible cause is slop in the linkage causes the
butterfly to flop backwards as soon as you retard past the magic
balance point... In combination with the above two it may also be that
there is a butterfly opening below which the swirl pattern in the
intake manifold suddenly collapses before re-establishing a new swirl
pattern...
Or, it could be just bad karma...

denny