Intermittent ping over VPN

[doubleduh]

At work we have a very strange issue. We have 2 offices in the same city on the same ISP, users in 1 office connect via PPTP VPN to office 2. The internet and local network in both offices is very stable. However, some users have very intermittent ping results (lots of timeouts) when pinging to servers in the other office. However, a ping to the VPN's external IP address is very stable. Also, some users have no issues at all. There seems to be no differences in the setup of the computers, same OS, same internet connection, same VPN settings (even tried with the same username). Resetting the routers doesn't help. The only difference I can think of is that a different route on the internet taken by the different client computers may be causing this. Could this be the cause or anything else?

 

[doubleduh]

Anyone?