H.264 files jittery

I'm using my mid-2009 macbook Pro to run quicktime files for a movie theater. Recently we started receiving these 30-second advertisements to play in front of the films. The best quality codec that I could get was H.264 files. For the most part, the computer has no problem running these. Most of them are under 500Mb and are 1080p. However, we recently got one that was just over 1.0Gb and has a good deal of slow-motion. Playing in either Quicktime X or Quicktime 7.6.6.(which I highly prefer to use because of the fullscreen preference options) the computer clearly struggles through playing only this file. The video becomes very jittery, but oddly enough, the audio plays through fine the whole time. I'm pretty sure it's a problem particular to H.264 because we run Prores at full 1080p will no problems, and those files can be upwards of 150+ Gigs. VLC does play the file better, but I'm really fond of the ability in Quicktime 7.6.6 to make fullscreen go only to the projector while leaving my desktop open on the laptop's monitor.

I have a Mac Mini that's about 6 months old, that plays the same file just fine. Now, from what I understand it takes some decent processing power for a computer to decompress H.264 fast enough to play correctly. Can I change anything to help this or is my computer just too slow? Here are the hardware specs:

Mid-2009 Macbook Pro

2.53 Ghz Intel Core 2 Duo Processor

4GB 1067 Mhz DDR3

NVIDIA GeForce 9400M 256 MB

OS: Mountain Lion 10.8.2

And a little bit of history for the computer. We were running Snow Leopard until Mountain Lion was released. There were not a lot of issues pertaining to playback quality, but then I updated the OS and started having trouble playing most full HD 1080p quicktime files in Quicktime 7. Quicktime X seemed to work fine but I hated the lack of preference options. The solution was to run the files from a 7200 RPM external harddrive hooked up via Firewire 800. The faster speed seemed to take away the stress from the computer that was giving it such a hard time. I only mention this to stress that I know the computer is more than capable of handling large full HD .mov files, but is somehow struggling with a relatively tiny H.264 file.

The playback is just as crummy regardless of whether or not the computer is hooked up to the projector or not and whether the file is sourced from the external drive or the onboard drive.

I would really appreciate any help, even if it's just a confirmation that H.264, quicktime and my computer will never get along. And sorry if this is a kind of rambling post.

M.