thanks for your suggestions and solutions.
internet bandwidth : 20mbps
machines : 77 units
no other surfing workstation, etc... the 20mbps is purely for mining machines only.
i found out there are few abnormal tcp/udp port in the network monitoring...
udp 123 (NTP)
port 995
port 4070
Do we need this for mining

Do you have this problem in all 77 units? If not, i doubt its a problem with your LAN. Unless you have badly made cable or have a switch issue that involves that specific group of miners.
Do test without any LPM modes enabled, and with another pool.
A single S9 needs like 1KB/sec, so 1mbps should suffice for 125 units...