BGP Flapping / Carrier issue

I am running a SDWAN infrastructure with one Hub and several spoke (15 spokes). All sites are equipped with FGT  clusters running 7.2.x. we have ADVPN configured with iBGP running over it for route advertisement .On 4-5 spoke sites  I have a strange behavior that is every 2-3 days   have this error in router events logs as below:

BGP: %BGP-5-ADJCHANGE: VRF 0 neighbor 192.168.88.254 Down Hold Timer Expired

BGP: %BGP-3-NOTIFICATION: sending to 192.168.88.254 4/0 (Hold Timer Expired/Unspecified Error Subcode) 0 data-bytes []

BGP: %BGP-5-ADJCHANGE: VRF 0 neighbor 192.168.88.254 Down BGP Notification FSM-ERR

BGP: %BGP-5-ADJCHANGE: VRF 0 neighbor192.168.88.254 Up

The problem is that during this event all connections from spoke to hub are shut during roughly 60 or 120 sec