Win98 clients fail to login

There are several Windows 98 machines that are receiving errors when logging in stating “Login request was denied by the network. Try again later.” These are Win98 clients logging in with client for microsoft networks to a NT4 server. I originallythought this was a symptom of network collisions, but when I used the administrator account to login in, I was successful. The account is not locked out of the server, and the error is not one that I had ever seen associated with user accounts and login. The problem is hard to track down because the scenario is not predictable. The clients will login correctly for weeks, then have trouble, which will clear up on its own after som,e time. Any help would be greatly appreciated.