We need a lot more information to be able to help you. First please fill out your profile and provide at least information, which box you are using, which image you are using, whcih version of the image you are using.

Without knowledge on how your harddrive is connected to your box, what model it is and what filesystem you use, we won't be able to help either.

Also: are softcams involved? Could it be, that other plugins or connected hardware eat up so much ressources, that recording or playing HD material doesn't work? Have you checked if the picture errors are in the material (i.e. recorded) or come from playing the film?