The Replication monitor reports the following for different DCs at different times:
"The following errors occurred while attempting to test replication. An error occurred while checking server <DC FQDN> in site <SiteName>. Error: Domain controller <DC FQDN> does not exist or cannot be contacted. The following domain controllers failed the replication test. See details below for more information"
This could be due to network issues or other communication issues with the DC.
Try changing the frequency at which the test runs to 5, 10, 15, and 60 minutes to see if the errors decrease or increase.