Network drives dropping out

> Vista Home Premium PC, "Server", has 2 shared folders. Another Vista Home
> Premium and a Vista Home Basic have drives mapped to these 2 shared folders.
> Everything works OK first thing in the morning but, sometimes within a half
> hour, sometimes up to 2 hours later, the network drives are no longer
> accessible. The programs that use these drives suddenly fail as they cannot
> access the data.
>
> If I go to the "Server" Vista Home Premium, I can access the shared folders
> OK through Computer, C:\filename, but not if I use Network and try to access
> the drives on "Server".
>
> All 3 PCs are Private networks, same workgroup, and are using the standard
> Vista firewall. I have tried mapping via IP address, rather than netbios
> name, but it makes no difference.
> Rebooting "Server" solves the problem for a while, until it reappears.
> --
> Doug