Navision randomly closing on clients

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?

Shucks! Meant to post this in the Technical foruum, I must have fat mouse fingers. Any chance of moving it? [:I]