Flappy Site to site VPN Sonicwall

Hi,

I have 2 Sonicwalls connecting 2 sites.  One is Sonicwall TZ500 , the other is Sonicwall TZ200.

The Site to site VPN between the 2 firewalls was fine for  years until recently, it becomes flappy. I checked the log and found these:

IKE SA lifetime expired. 

IKE Initiator: Start Quick Mode (Phase 2). 

IKE negotiation complete. Adding IPSec SA. (Phase 2)

Received IPSec SA delete request

Received IPSec SA delete request

I checked all the VPN parameters like Hash, Authentication, Lifetime, etc. all are the same.

Can you help me why it's flappy?

Thank you!