1) What is the normal speed for update 2 tables and 3-5 tables for getting info?
   <B>WITHIN ONE SECOND</B>

2) Which of the following are you using...
   Pessimistic or optimistic locking?
   Row or table buffering?
   <B>TABLE BUFFERING</B>

3) What RAM do the using PCs have installed?
   <B>HYUNDAI/LGS, 64MB in client and 256MB in server</B>

4) What is the network operating system?
   <B>USING WIN98 As OS in server with TCPIP protocol.</B>

5) Do you hear the hard drives running on the using PCs when the slowdown starts?
   <B>Not sure. I will check it out later. However, the mouse cursor in server will hang for a while if updating from client machine.</B>

6) Are you using transaction processing?
   <B>Yes.</B>

Your "my data may be not saved for my child table" seems to be a separate problem and we need more details about this too.
What info do you need ?

Does S.M.A.R.T., screen server setting and power saving in server machine effect the performance?