Site to Site VPN instability / Packet loss (fortigate)

Hi all,

Fortigate 140d running 5.07.

Site to Site VPN with 5 Local networks with matching phase 2's.

10 Azure VM's.

Has been working fine for a number of weeks until Wednesday.

Previously averaging about 25-40 millisecond latency across the site to site vpn,little to no packet loss.

since Wednesday, the performance has been very bad, dropped packets , connecting status almost constantly, latency of around 80-500 milliseconds..

I have been looking at the MTU/MSS settings as a start. 

1. Set VPN receive and Send MSS To 1350
2. Set internal interface MTU to 1350
3. Set Azure VM's interfaces to 1350.

All to no affect

what would you suggest as the next step ?

Regards Gavin Higgins Windows/CRM infrastructure Consultant