Cisco VPN through Fortigate VW

We have a customer with a IPSEC VPN(could be Cisco) through a fortigate virtual wire pair. The tunnel comes up and it seems after about 30min or so the tunnels drop where they are unable to get them back up. When we remove the the gate everything works fine. I have disabled NPU offload to keep the ESP local to the CP for traffic capture and processing purposes. All other services through the VW works fine. There are no MTU issues. There is not UTM or inspection done on the traffic, just a VW policy with action accept.