acceleration stumble when warmed up after idle
 

2002 neon base, after it warms up it will stumble after idling for a minute or so, bogs accelerating, but if I pull back into Low, it seems to accelerate better.

Is it possible something is preventing the transmission from dropping into Low after it warms up? I'm having a difficult time reckoning with that as when it finally accelerates it seems to up-shift correctly.