Ping interrupted periodicly with Fortigate routing

Hi,

I try to use the old Fortigate 200B like a router between two lans : the first lan (192.168.90.0) has access to internet and second one (192.168.2.0) is used for production. port 11 : LAN 192.168.90.0 (ip : 192.168.90.1) port 15 : LAN 192.168.2.0 (ip : 192.168.2.1) I can ping from a workstation in LAN 192.168.90.0 (ip : 192.168.90.138 , mask : 255.255.255.0, GW 192.168.90.1) to the second lan. but the ping is interrupted periodic for 3 or 4 seconds and then ping continue.

My question is why the ping in interrupted.

I need your help. Thanks in advance