on lan terminals loose connection to server from time to time

I have a server that connect locally to lan of mikrotik routerboard 1100 its ip is 192.168.8.128 …
There are 3 computers that connected also on the lan there ips is 192.168.8.125 192.168.8.130 and 192.168.8.131
when the computers (terminals) connect to server the stay connect for about 5 minutes then lose connection and take about 10 minutes to reconnect again
when i connect server and computer on lan before mikrotik (without mikrotik ) they work perfectly
any suggestions
Thank you