from what i've read paint tool sai will auto smooth lines like flash. i can confirm that art rage 3.5 and 4 have a similar feature. (sketchbook pro may smooth lines too, been a while since i've used it)
I tried everything I could think of, nothing helped. Flash is the only program that I've seen keeps the brush stroke selected until you are completely done with your stroke rather than just letting you make multiple strokes. It's apparently the processor speed so not anything I can do about it.