TCP Provider: The semaphore timeout period has expired.

Hello,

I have a branch office (Server 2008 R2) server running SQL 2005 SP4 (9.0.5057) that is getting the following error message

Replication-Replication Distribution Subsystem: agent BRANCH1-JOBNAME-HEADOFFICE-10 scheduled for retry. TCP Provider: The semaphore timeout period has expired.

It is replicating data over VPN to our Head Office server (Server 2008 R2) running SQL Server 2008 R2 SP1 (Cluster). The connection between the two serves is stable. The issue arose about a week ago but had been working for over a year before that. 

Everything I seem to read points to a network issue but I haven't been able to find a cause of it is indeed network related.Any recommendations would be appreciated!

Thanks!