Windows server losing connectivity

A Windows Server 2008 R2 (SBS 2011 really) with Service Pack 1 started to expose a network connectivity problem all of a sudden which seems rather hard to debug:

Occasionally (always during business hours, approximately 1-2 times a day) network connectivity is lost. The host itself keeps on running, I can use the console interactively. The Ethernet link as indicated by the LEDs in switch management and the NIC keeps being up. The IP configuration is still attached to the interface and looks valid (ipconfig produces a sane output). However, not even ARP lookups are able to complete successfully.

A list of things which did not help matters:

• resetting the switch port or the switch
• disabling / re-enabling the server's interface (either in ncpa.cpl or in device manager)
• unplugging / re-plugging the network cable

Shutting down and restarting the server always helps – it is usable as ever after startup.

Things checked:

• the event logs do not list any suspicious events
• the switch's network port counters do not show errors
• the network connection does not show any signs of queerness (losses, latencies, bad performance) as long as data is flowing
• the cable and the NIC have been replaced to rule out an obvious hardware failure
• power management for the NIC has been disabled in the device properties (device manager)
• the NIC used is an Intel PRO/1000 CT with the Intel 82574L network processor (the same type is used in the on-board NIC ports)
• the anti-viral suite from AVG (Anti-Virus Business Edition) is installed on the server, but the Firewall component has been removed when installing it, so it should not interfere here

As a hardware failure seems unlikely, I am trying to determine what software component could be responsible for messing up the network stack this badly. Is there any sane way to find out which drivers are in the network stack and thus possibly interfering? Has anybody seen anything similar before? Any ideas possibly leading to the resolution welcome.