Is it possible to have two or more instances of Quicktime player in use and not have "jerky" playback?

I'm attempting to execute a total of seven (7) discrete videos via QLab (which utilizes Quicktime for the playback). One video is fine, anything after that causes all windows to stutter during playback.

Current specs for test:
Mac Pro Desktop with (2) 2.8 GHZ Quad Core Intel Xeon
OS X Version 10.5.5 with all current updates
(2) ATI Radeon HD 2600

QLab cueing software
Test video is Digital Juice video clip at 1920x1080

I've also attempted to convert video to different formats (AVI, H.264, etc.) with no change in the playback. Not sure if the problem lies in the video card pipeline or Quicktime running in multiple windows. Several other individuals are working on this problem with negative results. Any help would be greatly appreciated. The answers are time sensitive, so the sooner a response could be given,the better.