Apey's screenshots show the issue pretty well I think - The header and advertising load up ok, but none of the actual forum does.
Well not really, just a hint really. As Coby says the advert is the first place to look.
The key is often in the bottom left corner where it shows a description of what is trying to load ie the a file name or URL. My first suspicion is always do with the incoming file. It can be an advert hosted elsewhere and therefore it is a server issue at the advertiser end.
If that's the case, there'll be something in there that is affecting certain users but not others. The ad server will be generating an ad from a particular host based on the whoever the user is.
If the iinet ad is the culprit, then we give it the flick, simple... and tell the agency to sort out the provider issues.