Sounds like there may be a momentary ping spike which may cause this, as opposed to AFK. How good is your internet, out of interest?
A few more questions:
1. Does this happen at other ferry ports, or is this the only place it happens?
2. Are you using any mods, or is this a vanilla profile? If you are using (non-SCS/MP) mods I would recommend you disable them.
3, Again out of interest, how good is your PC, are you able to provide any specifications?
As an aside I am not getting the issue, as I said it is most likely to be on your end. It is unlikely to be an antivirus issue.