Dear @sbickford,
Well, I would say that 10.6.8 is the problem. The newer playback system offered originally in 10.7, but much more mature in 10.9, is called AVFoundation. Isadora uses this automatically if the operating system supports it. If not, it falls back to the older system, QuickTime. But QuickTime offers poor performance for H264\. You'll have much better luck with Apple Pro Res tho. Try that codec instead.
You won't have to check the bandwidth for APR... only H264\. You can check the bandwidth by choosing the "Show Movie Inspector" option in QT Player.
Best Wishes,
Mark