As I mentioned in the thread Browser not working [Solved]
, another possible cause of this type of problem is the MTU. I've experienced this problem both with wired and wireless networks. For example, on my family tower PC I have to lower the MTU to 1492 in order to get SL to work with the wired network (it has a SiS Ethernet card), and today, during the attempted installation of SL 5.4 x86 Xfce edition (the installer crashes on my old Gateway Solo 9300, although the LiveCD works) I discovered that SL would only access the Web reliably via WiFi with the LiveCD if I lowered the MTU to 1464 (Linksys WPC54G v7.1 wireless notebook CardBus adapter with an Atheros AR2413 802.11bg NIC).
So the first things to suspect when you experience such behaviour are MTU and ipv6.
EDIT (Sun Nov 28 23:05:07 GMT 2010): If it is literally just one or two sites that won't load but all others load OK then the problem could be caused by a so-called 'black hole' router somewhere in between you and the site. See my post One Web site stalls (due to 'black hole' router) [Solved]
for the solution to this.