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Odd.
Normally XOOPS is using dynamic width, meaning it will go as wide as it can get.
Maybe you cahnged something in the CSS? From say width="100%" to width="600px".
Had a look at your CSS and nothing there seemed wrong - but I might have missed it.
What happens if you place a centre-left and a cntre-right column side by side as the only centre-elements?
Did you change anything in your templates?
In your HTML set to the page you have this:
Your width is not defined. What is it? pixels? Per centages? Don't know if it will make a difference, as I can't locate the exact syntax at the moment.
The W3 validator doesn't like your page. Don't know how grave the errors are though.