SP1 Causes Continuous Network Reconnects

SP1 causes a cycle of connects and disconnects on my wired network connection. Problem disappears when I uninstall SP1 but reappears when I reapply SP1. My connection is through an Intel Pro/1000 CT chipset on an ASUS P4C800-E motherboard going to a Netgear WGR614 v7 router. Problem remains even if I uninstall my Avast antivirus software. Just using Vista's native firewall. The network icon in the system tray cycles through a yellow warning sign going on and off. I've tried the repair connection option in Vista but it doesn't find any problem. I uninstalled the driver for the card hoping a reinstall would rebuild and fix the problem but it made no difference. I'm using Microsoft's drivers for the NIC chipset. I power cycled the router but that made no difference. But as I said, uninstalling sp1 restores a normal network connection. An ipconfig when the connection is good shows an IP in proper range for the DHCP from my router. At the disconnect, yellow warning sign, it shows no IP. I've attached entries from the system log that illustrates what happens in a reconnect cycle. I deleted the XML event data for brevity but can add it to this thread if it's necessary.