Continuous disconnection in RDP over IPSEC VPN

We have a fortigate 200D with v5.6.11 build1700 (GA) FortiOS version. The VPN connection is stable but the RDP over IPSEC has Continuous disconnection. Of course until the number of vpn connection is less than 6 or 5 connections this issue does not occur, but by increasing the vpn connection the RDP encounter with disconnection every 2 to 4 minutes.

Also Ping to VPN fortigate  never  timeout, but I get disconnected (or reconnecting) from my RDP sessions every minute or so.

The following changes was applied but they have not been effective: