Lotus notes client and nav 3.6 not playing nice

I have a client that is running Lotus notes R5 Client and Navision at the same time. 3 out of 10 computers are locking up and need rebooted from this unknown error that is not logged in the event viewer of the computers it starts out fine then about 20 mins into the apps running navision slows down to a craw you list in navision and the redraw is one line at a time eventually navision stops responding and the computer crashes