Airport Message "can't find server"

I have a wireless network utilizing the time capsule which works well for all uses such as remote printing etc., with one exception. When I try to update any program I get a message after the program begins the process stating that airport cannot connect with the server and suggests I do a network diagnostic. When I do the diagnostic it indeed does suggest that I cannot access a server even though I can access the internet. It also will not recognize my time capsule password during this diagnostic. When I do a diagnostic at another time when I am not trying to download an update it does show that I can access the server beyond the internet. Any ideas on this peculiar problem.