I don't think you can time out on processing an individual
pixel, but I could be wrong. You could try reducing the number of
reflections in the filter and seeing if that fixes the problem. It
could be a math precision difference between the cards.
Shaders can (and will) time out, but individual pixels
shouldn't. It could also be a driver issue with the structure of
the filter. I have a x1600 mac book pro here and I'll try it out if
I get a chance.