stuttering playback

I'm doing a project for my job in which I have created some video transitions in photoshop and brought them together with 5 pre-existing quick time video clips in After Effects 7.0.

The QT clips play fine on their own (before After Effects), and even playback fine within the program with a RAM preview, but after I render everything out, the QT clips (now all together in 1 project with the transitions), stutter or jerk when I play the project back.

It may be important to note that:
This stuttering only happens on 2 of the 5 clips;
There's been no editing done to the original QT clips at all, they
are just dropped into AE and used "as is"; and
Although the file is large, (145kb), this PC is new and space is
not an issue.

Does anyone have any ideas on how to fix this? If so, please explain things on a beginner level as I am still new with After Effects. Any help at all would be GREATLY appreciated.
Thanks,
Angie