IPSEC VPN - critical event - Invalid ESP packet detected

[#1]

hi all, i have setup policy-based VPN to connect my primary site to secondary sites. Every sites have 2 fortigate 60B with fortios 4.0 mr1 patch 3 in HA active-active Primary site have 2 wan inteface connected and i have policy-base route to make VPN priority on wan2 The VPN connections comes up regular but often it becomes very slow and we have to reboot the firewall on primary site. I receive log from my firewall with critical errors like this: Message meets Alert condition The following critical firewall event was detected: Critical Event. date=2010-04-14 time=12:17:30 devname=FGT60B3909602607 device_id=FGT60B3909602607 log_id=0101037132 type=event subtype=ipsec pri=critical fwver=040003 vd=" root" msg=" IPsec ESP" action=" error" rem_ip=85.18.xx.yy loc_ip=85.33.zz.ww rem_port=500 loc_port=500 out_intf=" wan1" cookies=" cb2777639e5aa8e4/e738e249d3f79d56" user=" N/A" group=" N/A" xauth_user=" N/A" xauth_group=" N/A" vpn_tunnel=" NAtoRO_wan2_tun" status=esp_error error_num=Invalid ESP packet detected (replayed packet). spi=1109cb9a seq=00000195 Can someone help me to resolve the problem ?