VPN | Traffic down for few seconds and comes up | Tunnel state not down

Hi Gents,

I am a new bee in the FTNT forum.

I have a Fotinet 1500D with firmware version V 5.2.10 build742. I have many site to site VPNs connected i have put a ping monitor to the other end . frequently i get traffic drop messages, but once i check tunnel monitor in the box its not down traffic is dropped and it comes back . please advice appreciate your help. 

other end device is also 1500D v5.2.10,build742