Cisco Routers :: Error Using PPTP Pass-through In RV180W?

I am having a problem connecting to my office VPN from home since I changed my (home) router to the RV180W. I am not using pre-configured tunnels, I just connect to the office VPN using the built-in Windows PPTP client when necessary.  I have a small network at home using the router in a NAT configuration on my static IP fibre DSL connection.
 
I had previously been able to connect using PPTP from either XP or Windows 7. Now I get "Error 732: Your computer and the remote computer could not agree on PPP control protocols".  Unfortunately I don't know what hardware or PPTP server is used at the office.(I don't know if it is relevant but I have also configured the RV180 PPTP server to allow incoming VPN connections - this works fine. It doesn't make any difference to my issue whether it is enabled or not.)Are there any known problems using Windows PPTP client through the router in a NAT configuration.  Is there anything I need to configure apart from enabling PPTP passthrough?
 
Edit: Just to clarify, the problem is making PPTP VPN connections from XP/Win7 on my home network over the internet to my office VPN/PPTP server, via the RV180W as my home router (using NAT/PAT).
 
Edit: More info.  Looks like the office VPN is a Microsoft RAS server on Windows 2008 behind a cisco PIX firewall.
 
Edit: Just to be sure, I swapped out the cisco RV180W for my old netgear router and I was immediately able to VPN again with no issue from both XP and Win7.  When I swapped the RV180W back in again, I was able to VPN from XP but not Win 7 so it looks as though the PPTP passthrough works occasionally but is intermittent. Just to be clear, the VPN is only intermittent using the RV180, it has always worked flawlessly on the other router so I don't think there is a problem with the internet connection or client/server configuration.