I noticed that if I were to reduce the speed on 24P footage, it tends to end up not smooth and jitters from slow to stop to slow to stop... not the smooth slow motion like a milk commercial
Is it because the footage is 24P?
thanks
Yes, it's because of 24p. The most popular solutions, not necessarily the cheapest, are to get either Twixtor or Boris Continuum with the Optical Flow filter.
You can shoot in 30p for free, though, and try slowing your footage that way. It does lose a little of the 24fps 'feel', however.