XP Mode in Windows7 Professional Drops Network connectivity

Question

Ok - I've seen and followed several (5+) discussions on this problem dating back 4 years through Vista, but no one seems to have posted a definitive question.  I've been rolling out Windows 7 for over a year now and have been following the same procedure
and installing the same apps to the same configuration without issues until this week,  I now have 2 boxes doing the same behaviour.
Environment: 
Workstation is a PowerSpec B634 with Intel motherboard, using onboard RealTec NIC.  Windows 7 Professional, with Microsoft's Virtual XP vhd.  They are connected only via the ethernet NIC (wireless is disabled) on a 1GB connection.  I've moved
the port on the switch they've been connected to and switched both the patch cable at the rack and the ethernet cable at the workstation.  And, as I've said I'm seeing this behaviour on 2 separate boxes.
Behaviour:
Unit boots flawlessly and I've not seen any connection issues on the Windows7 side of the world (i.e. Outlook isn't reporting disconnects from Exchange and the NIC isn't reporting loss of connection.) However, in the virtual XP side I can launch the virtual
XP, and then start my database application.  It finds the database network share (mapped through the virtual xp as a drive letter to "//tsclient/drive" referencing a share on the host Windows7 machine.  The application runs great up to
10 minutes at which point the connection to the network drops as the database client loses all pointers to it's tables and references and crashes.  If I start a remote desktop session from inside the vitual XP it also will run fine for a while then report
the conenction has been broken.  At no time does the NIC icon in the virtual XP system tray report loss of network connection.
I have turned off all power-saving settings in BIOS, Win7, and as much as I could find in virtual XP - - I'm really at a loss here, especially as I have 15 other machines of very similar specs and identical software installations working just fine, oh, and
yes, I'm running SP1 and have downloaded all available patches.
Hints tips suggestions and/or more questions all very much appreciated.

Answer 1

Thaks for the suggestions - as noted in my original post:
1) all power management for the NICs (which are integrated on the Intel Motherboard) have been switched off in BIOS, Power Management in both the Win7 host and XP virtual environment, and in Device Manager specific to the NIC I've switched of any power management
options to force them to stay on at full speed/power at all times
2) I have had this problem now with the computers plugged in to two different "patch to outlet" locations in the office (i.e. different in-wall wiring), plus have used two different wall to PC ethernet cables (which I have also pin verified with a fluke
cable tester) and have used different patch bay to switch cables and have not only switched ports on the switch but have switched to a completely different switch.
With those considerations taken into account I am continuing to see the issue.  The connectivity drop is occurring pretty regularly between 6 and 10 minutes of operation.
3) sorry, yes, NICs are same model as both towers have same Intel motherboard.