login timeout

I have been testing Server 2008 R2 RemoteApps. If I leave a program up for
a while, it eventually goes to a screen that asks for my password. On the
Terminal Server, the session still shows as Active, so I don't think it has
anything to do with session timeouts (which are all set to Never in multiple
places as I have been troubleshooting this). So far, I don't see a
consistent amount of time before this happens. It will definitely happen if
the program is left up overnight, but I've had it happen during the day as
well. I have turned off the power management on my network card to try to
eliminate the possibility of that breaking the connection and causing this.
(My workstation is Windows 7, but this also happens on XP.) What other
possibilities are there to prevent this from happening?
Other info: Terminal servers are virtual on Hyper-V. Domain controllers for
Active Directory domain are Server 2008.
Thank you in advance for your assistance!
Angie