Sync issues after some time of playing together

[PC Steam][0.90.11][Sync / De-Sync] Sync issues after some time of playing together

My friend can join and play normally, but at some point (sometimes soon, other times takes a while) he will de-sync and can not interact with anything.
To fix this, is for him to reload back in to the instance or sometimes I need to relaunch the instance again for him to be able to join.

Is there an specific game port number, besides the standard Steam Ports (TCP: 27015-27030, 80, 443 UDP: 27015-27030)
that perhaps I need to forward from my router to my pc to help alleviate this?