Navision randomly throwing users out

Hello I have a customer running Navision 2.01 on the native database. They are running NT 4.0 and nine client machines are Windows 2000 and one is Windows 98. They have 10 users concurrently logged in on a 10 user license. They have been experiencing, on some machines, Navision closing down (disappearing). They receive no warning, or error message, and can log back in without problems. The close downs seem random but can happen up to three times a day, which is annoying. Does anyone have any ideas?

Did you check the TCP/IP Settings(keepalive time) for PCs having trouble? or its random and across the network?

hi Mohammad I have verified that the connection is TCP to the database from the clients, how do I verify the keepalive time? Yours - a non-technical person [:D]