Often times we find that this is the result of the server that is running PRTG syncing to a time server which causes PRTG to "lose data" for that time period which causes the gaps seen like above.
If this server is constantly having the time changed on it by the NTP server, you can try removing these settings so it no longer syncs to an NTP or you may need to get the clock repaired so that it isn't constantly being reset. Also, if this is a VM server, if the host is also synced to a NTP server, then this may happen so check those settings as well.
If this is a problem, you will see many errors in the Probe log that say Timediff: and the number of miliseconds after it. If you see those repeatedly in your Probe Log, try the steps above. Where does PRTG store it's data
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.