My 2 cents worth:
If a lot of PPRuNers are indeed using "tab-capable" or other similarly automated browsers, then wouldn't that be intensifying the problems?
I'm not an IT pro, but if I understand the principle correctly, these auto-browsers would automatically download "adjacent" pages or links, which may in fact never be viewed. If 1 PPRuNer's auto-browser is downloading upto 30 pages at a time including the 1 currently being viewed, then surely that is the equivalent of having another 29 "virtual" users online. All of whom are consuming processor capacity and bandwidth.
I have an ADSL connection and to my knowledge, no ISPs charge on the basis of data volume, except for certain services requiring dedicated satellite resources such as Inmarsat. Conversely, a significant proportion of the costs of hosting a commercial website are based on the volume of data.
Perhaps there is a way to limit the number of concurrent connections from a single PPRuNer/IP address so as to limit abuse?
Also, the vBulletin / MySQL software used is very competitively priced...and whilst offering a plethora in the way of wonderful features, the question has to be posed. Would spending more on the software be more economical in terms of computing resources and bandwidth?