The VPN tunnel goes down

[#1]

I have a FortiGate 50B firmware 3.00-b0730 (MR7 Patch 1) with 10 VPN IPSec fully functional (to Cisco devices, jupiter etc.) of my clients, I migrated the VPN to a FortiGate 200B firmware v4.0, build0303, 101214 (MR2 Patch 3) with the same configuration, but i found numerous problems with some device vpn for example with a Cisco ASA 5520 with software release 8.2 (2). At first, the tunnel is established perfectly, but after a some hours, the tunnel goes down and I have to reset the tunnel. Why? What has changed from version 3.00-b0730 (MR7 Patch 1) the fully functional version v4.0, build0303, 101,214 of the FortiGate? thanks

[#2]

I tried with your advice but the problem remains .... Now are in contact with the customer service, and as a workaround i have enable the DPD in both direction FortiGate/Cisco, so every time renegotiating the key, the tunnel goes down for 20 seconds and then goes up. I hope the fortigate developer solve the problem...

[ede_pfau]

Can you live with that short interruption if you set the key lifetime to the max?

[#4]

the problem is only one, I have a call center 24 hour 7x7 and in this tunnel VPN there are calls and the web services...the interruption creates problems for the VoIP calls.