File Sharing on W2K Server

We are having a terrible time sharing Excel files on a W2K server. Clients are running Win98 and accessing files from the server starts out fairly fast. As the day progresses, the time to open and close a shared file gets slower and slower until it becomes almost impossible to open or save the file. I have checked all sharing parameters at the server level and file level, but nothing seems to help.

What am I missing? Sharing files has almost become an impossible task on our server.