Windows 98/2000 file sharing and caching

I've got a Windows 98 system sharing a file being accessed by both the 98 system and a Windows 2000 client(not at the same time). We are having issues where the 2000 system opens a file on the 98 machine, modifies it and then saves it. The 98 systemthen opens the file and the changes are not there.
I've disabled all disk caching on the 98 system and all caching and opportunistic locking on the 2000 system. The problem still occurs about 20% of the time.
Anyone have any suggestions?