I hate to sound like a broken record but I am also having playback problems with HD material in CS4.
I have Avid, CS3 and CS4 and Avid and CS3 play perfectly but CS4 is basically worthless. Under cS3 I have been able to get 3 streams of full HD playing back in realtime. Under CS4 I can’t even get one.
3 streams compared to 1/2 a stream is a prety huge drop in performance.
I have a quad core AMD system with 6 GB of ram and a ATI 4670 video card with 1 GB of ram.
I have tried Windows XP and Windows Vista 64 and it works exactly the same under both. I have tried it with CS3 still installed and with a fresh reformat clean OS install. I have wiped my harddrive and reinstalled various versions of Windows 5 times now and there is no change.
I have tried 4 different ATI driver versions with no chnage at all. I have also tried chnaging the performance level in the graphic card drivers to every setting possible and there is no chnage in the system. Full quality and draft mode playback in CS4 seems to perform about the same. My cpu also peaks out at 100% for a single stream playback.
I would buy more copies of CS4 if they could just get it to work. Adobe doesn’t seem to be able or willing to fix this problem because they will not even admit it is there.