It's an 'occasional' issue in that it chooses when it wants to render correctly or not. Most often it appears fine but then just as you think it's fixed itself, bosh, stretchification!

Pressing F5 from time to time doesn't bother me but a couple of visitors have commented. A complete noob might take one look at it and assume it's a really naff site and move on.

You might run the page(s) through these HTML and CSS validators. Sometimes that will identify problems.

http://validator.w3.org/
http://jigsaw.w3.org/css-validator/

In your original/theme, tables are set to 100% width .
When there is no content in the centre _block that could (but not necessary) just happen - stretching all the way across the browser.

It should be all in the theme. Play with the width of the tables and blocks, or try reorder the blocks and see if there is any change in behaviour.
what ever you do, test it on several browsers. mainly IE, FF, netscape.

I am extremely grateful for the replies, honestly. It's very frustrating for a beginner who is bogged down with a technical problem beyond his ability. It's only with a more responsive and communicative community that such a valuable peice of software as XOOPS is, will continue to grow, expand and reach out to develop it's userbase.

A certain member, by the name of Dana, approached me and suggested that a particular module was to blame for the block stretching. The culprit was the CafePress module which felt the need to take up the entire page when not displaying a recent product. A new product is displayed upon refresh but every once in while, it came up blank which in turn, resulted in the stretch.

I have now removed the offending block and all appears as it should.