Slow read speed?

Hello,

I'd like to ask a question about a chronic problem: I have a fairly good configuration (Quadcore 6600, 2 Gb ram, 8800GTX) but it isn't able to play apple quicktime movie trailers (HD quality ones) if I try to run them from DVD's (given by a computer magazine). The playback starts smoothly but after 15-20 seconds it starts to skip frames or freezes. The driver light blinks very fast during these episodes. My DVD driver is LG H42N, which is a good DVD writer. If I copy the file to tha harddik there are no problems at all. The driver is on UltraDMA 5 according to the device manager, so I don't think it is a DMA problem. My question is: Is it normal (which I think would be bizarre) to skip frames or freeze if you try to play HD trailers from DVD driver or is there anything I can do to speed it up.

Thanks.