Something changed recently either in your AE settings or on your computer.
A common cause for AE problems would be incorrect memory & processor settings when using multiprocessing; since you have 8 gigs -- not exactly a boatload by AE standards -- using multiprocessing could be something of a crap shoot in the first place, and you should probably avoid using it. Another common cause of woe: using Open GL to accelerate rendering. Don't do it, period.
If you changed one or more things on your computer recently, you'll have to track the changes down and report back.
Whether you succeeded in rendering to the Animation codec AFTER your H.264 woes began isn't exactly clear. But you can always use Adobe Media Encoder to make the H.264 file from the Animation file.
> When I RAM preview my final composition in AE, everything looks great, syncing is fine, etc. However, when I render the composition with the H.264 codec, and play it back on Quicktime there is an extreme amount of stuttering all throughout the animation--during fast moving parts and slow moving parts. <
RAM preview is not comressed to H.264 so of course it willplay perfectly.
H.264 is REALLY compressed but playing it back requires some beefy processor, especially if you're trying to do full 1080HD. As Dave says, go to Aniation and use AME to do the transcode. This has many benefits but mostly you're using a compressor applicaiton to do the compressing. And you have a completely perfect copy of the project form which you can create new copies in any codec or format. You cannot easily , for instance, use AME to move form the H.264 to WMV.
Brings up another question: why are you using H.264 at all? I'm asking because 90% of the answers we see to that question are incorrect from a technical point of view and inappropriate from a workflow POV.
Perhaps some graphics acceleration issue in Quicktime itself or on a more generic level in the operating system/ driver. Tried installing a newer (Beta) driver from NVidia/ AMD? Did you install any new Quicktime components lately? Changed something in your configuration? Could even be as trivial as plugging some external USB audio device into a different port...