What compression format are you using? It sounds as if the computer you are playing them back on can't handle the size of the movie. I generally render my finals at high quality, for example, Quicktime Animation codec at 100%, then export a smaller movie compressed to H264 for computer playback...
Regards,
JD
John Dickinson
Motionworks
Making it Look Great 1
Design and Production Techniques for
Adobe After Effects and Zaxwerks ProAnimator
Available Now