It is unlikely for an Ae composition of any degree
of complexity to play smoothly in the Source Monitor.
And in most circumstances for an Ae composition to play smoothly
in a Premiere timeline you must 'Preview Render' the timeline first
by selecting the timeline and hitting 'Enter'.
Once rendered, your timeline will have a green line above,
and barring hardware issues should play smoothly.
Premiere Pro CS5-CS6 Reference
(.pdf 12 MB download)