Not sure really. It's an odd one.
If
everyone was having the same problem, then I'd be looking at the server upgrade. But they're not.
If it was the same
link in the forums that everyone had a problem with, then I'd be looking at vBulletin to see what may be causing it. But it's not.
Different people are having the same problem (
php files are being offered for download instead of displaying in the browser) with different browsers, and different links.
The fact that most of the site it working for those who are having this particular problem, is even more perplexing.
It'll keep chipping away it.
Cheers,
Shaun
PS. Thanks for testing it with me, the feedback helps.