Problem with Lotus Notes 5 / Win NT 4 SP

I have a NT server running Lotus Notes 5.0.2c. Several times a day it shows a message “Note item not found” and after this, it starts using more memory and processing time until no body is able to use the server until the server is shutted down. In the statistics the memory usage grows as the number of users grow also. Does somebody have an idea or the solution to this problem???