vcenter and esxi host disconnects intermittent

Hi Experts,

We have vcenter and esxi as below

VMware vCenter Version 6.5.0 Build 4602587 using Windows

VMware vSphere ESXi, 6.5.0, 4564106

(upgraded from vCenter and ESXi 6.0 GA)

Server using Dell R730 and Storage using MD3800f

Recently we found strange behavior.

Vcenter keep sending alerts that he cannot discover one of the esxi, but the duration is around 1-2 minute. Then it comes to connected again as below log

I try to increase the heartbeat as VMware Knowledge Base  (the value is 120) but seems the error still continuing.

There is no physical changes (no cables disconnected, no people working around the server)

Could someone advice on this issue ?

Thanks a lot.

Regards.