I've battled this problem since mid November, I never had no problems until then, using Vista 64. What I did, which helps get rid of most of this for me, if not all of it, was reinstall QOS Packet Scheduling for my onboard Nvidia Gigabit lan, as before I didn't have it installed.
From what I have seen on my own system, is that Vista needs QOS packet scheduling to maintain a stable connection, to your router if you are using a router, which has a built in firewall.
Contrary to what other people on the internet, websites, and magazines say, having QOS Packet Scheduling installed and running, does not use the reserved bandwidth, unless you are running a QOS enabled application.
I found this out, in the Microsoft knowledge base. What surprises me, is that I went around with this with my cable ISP, and the idiots on the phone, couldn't even tell me to reinstall QOS Packet Scheduling, and input the domain name, and DNS server numbers in to my router.
I hope none of you uninstalled QOS Packet Scheduling from your nic properties.