Random speed loss

After a fresh install of Vista 32bit, I've recently installed the drivers for my speedtouch 330. I've noticed that I get my usual download speeds of 1.5 - 3mb/s at times, then it drops dramatically to 0.2mb/s for extended periods of time. My upload speed seems to be unaffected.

I'm really not sure where to start looking into it. I've uninstalled and reinstalled the drivers, different driver versions and back but it's not seemed to have made any differrence. Sorry I can't provide any more info than that and I'd really appreciate any advice.