How are you playing back the export?
What model CPU in your system?
Well that was a good question.
I WAS using windows media player. It plays OK in VLC.
When I test the original clip with quicktime player its a real mess
Much worse than in WMP
I reexported it with the target and maximum bitrate increased, and this version plays fine in VLC and WMP, but is still jerky in QTP.
I've got an i7 920 with 12gb ram. Its playing of a single 2tb hitachi drive. (which is about 3/4 full).
Yeah, WMP and QuickTime are about the worst players out there.
Stick to VLC and you should be fine. KM Player, GOM and UM Player are also quite workable.
About practical minimum for editing is three separate disks, one for OS & programs, one for media and one for cache,previews, renders and exports. See http://forums.adobe.com/thread/662972?start=0&tstart=0