It seemed to work OK for a few hours yesterday, now it's back to doing what it was doing.

I haven't tried on my Mac, it's on my PC where it's more prevalent... Windows 8.1 x64, latest Chrome.

Web Inspector is telling me this:
ubb_jslib.js?v=7.5.8:59 Refused to set unsafe header "Content-length"
ubb_jslib.js?v=7.5.8:60 Refused to set unsafe header "Connection"
http://fordfiestast.co.uk/625.jpg Failed to load resource: the server responded with a status of 404 (Not Found)
ubb_jslib.js?v=7.5.8:59 Refused to set unsafe header "Content-length"
ubb_jslib.js?v=7.5.8:60 Refused to set unsafe header "Connection"
ubb_jslib.js?v=7.5.8:59 Refused to set unsafe header "Content-length"
ubb_jslib.js?v=7.5.8:60 Refused to set unsafe header "Connection"
ubb_jslib.js?v=7.5.8:59 Refused to set unsafe header "Content-length"
ubb_jslib.js?v=7.5.8:60 Refused to set unsafe header "Connection"

Even better than screenshots (as they won't show you anything), a screen capture.

https://www.youtube.com/watch?v=-s454CSzBo8

Looking back at the video, the main index is a different width to the individual forums, there's just no consistency in the width - the gaps along the top of the forum between the header and the random image on the left are a bit strange. If the title is long enough and you click on the left of the topic title it will go straight in, if you hit the middle or the right of the topic title the forum width expands then you have to click again.

Very occasionally, some forums automatically fill the width when you load the subforum index but not before appearing as a thinner version.

My suggestions would either be - a properly implemented fluid width layout, or a properly implemented fixed width layout, at the moment it seems to be a mix of both which doesn't appear to be working properly.

EDIT: The banner isn't central to the forum layout and the random image on the right is aligned as far to the right as possible which is only in alignment with the main forum index, it's out of alignment and offset where-ever else you are in the forum.

My personal view would be to ditch the random photo element, create a new table row above the breadcrumbs and use that for the header rather than sticking the header into the body element.