It appears to me that the difference between your site and Tristar's is the size, and type, of the graphic.
Tristar looks to have used a graphic that spans the entire area of that image, I can't see the code, but yours appears to spane 1/8th of your chosen area.
You can do this but:-
Your gradient is not exactly vertical so you can see the join where it repeats, which makes it appear 'blocky'.
It looks like it's image related, rather than code related.
Does this sound correct?
If not, please provide more details.