screen rendering errors in Preview

I'm having issues viewing pdfs using Preview and quicklook: parts of the image are missing. The text is correct and jpgs are okay. The broken image only seems to affect the photoshop file linked in the original indesign file. The pdf renders on screen perfectly fine using Acrobat and print outs are correct too. This is really problematic because my clients don't always have acrobat and, when they double click a pdf it will associate with Preview for them - so it needs to be picture perfect in Preview (and Quick look).

I've already tried deleting the cache files associated with Preview (the ones I could find anyway). This didn't make any difference.

I've tweaked the preferences in Preview and this made no difference either.

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Snow Leopard 10.6.8 on an older intel imac (2.66GHz intel core 2 duo 4Gb 1067 MHz DDR3) but I don't have the specs for my clients' macs.

programs used to create layout and pdf: Indesign, photoshop.