Suddenly my browsing has become very erratic and jerky. What is wrong?

My PC has been running fine until very recently when I noticed that while browsing the internet my PC becomes quite jerky. The system freezes momentarily and then continues. This happens very frequently. (Can be several times a minute.)

I use IE6 and ADSL. The ADSL seems fine when using other software. I only notice the problem when using Internet Explorer. The only change I have made to my system recently is that I have installed a USB 2 PCI card and hub. One interesting issue is that both my PCI ADSL modem and PCI USB 2 card use IRQ 11. Could this be the cause? If so how can you force Windows XP to use a different IRQ?