Hey Will,
I believe you might be rendering in an uncompressed avi or a quicktime with “animation” codec and your hard rive might not be able to keep up with the data rate. I used to have this problem and then I setup a raid with two sata 150 harddrives and now it plays very high resolutions flawlessly. Even my G5 at work stumbles on very high resolution animation codec renders. One way to get around this is to render with the “sorenson 3 codec” unless you don’t have any alpha channels in your scene. At work I usually render with the “PNG” codec, the files are smaller and it supports millions plus with alpha channels. The playback is a little choppy but in a linear editor it plays fine.
Good luck
Malvin