NFS upload speed decreases after 40mb copied.

When I mount my NFS exports from my SuSE Linux 8.0 workstation I have a strange problem when uploading files to the NFS server. With all files bigger then 40 MB the upload speed decreases from 60 mbit to 10 mbit when 38 MB of the file is copied. With downloading from the server I have no problems and the network speed is fast. I read some NFS FAQ's and MAN pages and changed some mount options (rsize/wsize for example), but that doesn't work. My server is running SuSE Linux 7.3 with the knfsd package installed which is distributed on that distribution version DVD. Does someone know if this is a known problem? (and how you can fix it offcource ;-) With kind regards, Joep Verhaeg. http://www.joepverhaeg.nl