IPSec tunnels keep dropping - won't come back

Hi all,

We are having a problem with one of our Fortigate 80E firewalls and the IPSec tunnels we have set up to our other locations and for the life of me I can't figure out what is happening. It started when we deployed a new office and rolled out a pair of 80E firewalls.  We use IPSec tunnels (not in Interface Mode) to create connections between all of our offices.  This has worked for years.  However, at this new site we started to notice that some of the tunnels would drop randomly.  The issue is that the only way to reconnect them is to delete the tunnel and re-create it.  I've tried to re-do the shared key and delete and re-create the phase 2 connector, but only a full recreation of the tunnel will allow it to connect again.  

I thought at first it was the firewall, so we replaced them with a brand new pair... but the same thing is happening.  It is only happening at this one site and as soon as I recreate it the connection is re-established, so it does not appear to be a connectivity issue with the provider.  

I am at a loss... has anyone seen anything similar before?