Macintosh session time outs in NT domain

I have some users who are using Mac’s for some graphics work. We have Mac service loaded on a NT 4 server with SP 6a. Our users say that when they save a file to ths NT server the first time it works fine but each time after that it says that the file does not exsist. This message continues with attempted save until the server is rebooted. The event log shows that the Mac session has timed out and was disconected by the server. Is this a setting we can change? Is there something else going on? i try to avoid Macs if at all possible so I am not real familiar with them or how they work with NT.