Wavy Lines in vinyl siding

I have a question I shoot a lot of houses for a local magazine. With some of the houses with vinyl siding when I get back to look at the pictures they look great but when the picture is rezied or in a thumbnail view I get all these dark wavy lines through the siding on the house. Has anyone seen something like this. Thanks.

That happens when it is a full size pic that is thumbnailed.....If you could resize a pic to the thumbnail size and then link to the bigger pic it would stop that!?!?

could also be aliasing in a crisp small photo. post some examples here so we can see... maybe try blurring the image just a bit. the problem is just that if you have a very crisp diagonal line, you can often see the individual pixels. antialiasing it (blurring) a bit smooths out the edges.