My term server proxy auth stops working after several days.

After rebooting my Border server and citrix server, the term server proxy
auth works fine, but it stops working after several days (or some other
random event - maybe number of connections through border?).

Last night I rebooted my Citrix server, expecting this to fix the aut
problem, but the term server proxy auth still wasn't working. So I
suspect it is something on the border server.

T get by, I have a rule that allows members of an NDS group
called 'specified sites' out to browse to a given list of sites.
I've added the citrix IP address into this rule as permitted as well, but
this is limiting for some people.