Sorry to hear that, certainly is an odd error. I had a similar type of problem where their was an issue with the wishlist interferring with adding to cart, but it was on all products, not select products, which makes this weird.
I would be happy to look into this for you, and if it turns out it's a bug on my part, cost would be free, and if the bug is on your or x-cart's part it would be billed at the hourly rate.
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