As far a I can tell it's the large sky ad, the banner one seems ok but the sky one that is put on thread pages after a couple of posts seems to cause the slow down.
Actually, the problem only seems to occur when you have the large square sky £17 a month on the same page as the banner sky £17 a month one... maybe they share some resource?
It will be one specific advert which loads in that box, rather than all of them. next time it happens let me know the specific advert which is in that box, and i'll pass on the information about the likely culprit.