out of interest, how long is the clip that you are trying to play? Can you see a green render bar? Has any RAM preview been generated?
What about the playback feedback figures? That is, what does Motion say your frame playback rate? Does it look erratic?
You also say the footage is HD. What kind of HD? What is the resolution? Where are the files stored... are they on the same internal hard drive or are they on an external? Is the external a USB or firewire drive?
Lots of questions :)