I was wondering if anyone has seen this at all?

I have a G5 Mac OSx 10.3.2 with 1.8Ghz Processor and 512 Mb Ram accessing a
NW6 sp4 Cluster using Native File Access.

Now what's happening is that if the Mac is left for a period of time, say
15-20 minutes, the user has to re-login to the network as it is has
disconnected him. Is there a timeout set anywhere that I have not seen?