Do you have any other netflow flow sensors in PRTG? If so do they have the same issue? If not, would it be possible for you to pause this sensor and create a new sensor for this port and see if that also has lag issues?
Is the time on the target device correct?
Would it be possible for you to send over the probe state logs?
Please go to the "Setup"->"System Administration"->"Administrative Tools"-Page in PRTG and click the button "Write Probe state file". The state file(s) should be written into the "Logs (System)"-folder, please send us the content of this folder.
Please also forward us a screenshot of the overview page of one grey sensor, a screenshot of the Live graph and live data table of this sensor and a screenshot of the "Setup"->"System Status"-Page in PRTG. Then use the option to upload the state files and the PRTG System logs to our FTP-Server, using the button "Send Logs to Paessler..." on the tab "Logs" in the "PRTG administration tool". You can do this from the Probe that runs these sensors (Important if you are using Remote Probes!). Please refer to this kb-article.