PPTP through NAT fails

LAN -> Linux Router -> Internet -> PPTP Server
NAT

192.168.90.1 -> Linux Router -> Public IP -> Internet -> PPTP Server
NAT

Kernel: 2.4.34
Iptables: 1.4.12.1
Chain: INPUT ACCEPT
Chain: OUTPUT ACCEPT
Chain: FORWARD ACCEPT

Problem: PPTP connections fail to certain PPTP servers but only when being NAT'ed

Using the stock windows xp vpn client I can connect to a windows SBS 2003 server using the PPTP vpn connection whether using a private IP
that is being NAT'ed or a public IP. However tring to connect to a Nortel PPTP server fails when using a private IP that is NAT'ed at the LINUX router. For kicks I've thrown in an off the shelf router that is NATing its traffic and it works OK connecting to the SBS Server or the Nortel PPTP server. Based on the above tests I assume something is happening during the NATing process that in some way causes the Nortel PPTP server (and several others) not to make a successful connection.

Looking out on the forums I see a lot of people referencing trying to tunnel back inside the NAT gateway but not any posts saying they were unable to connect from the private side to the public side.

I've seen references to a PPTP_NAT helper module but once again this appeared to be from going from the outside in not the other way round.

I'm really just looking for a best direction to try next in the trouble shooting process or fishing for someone who has had a similar issue and says "Oh yea I had that problem you have to do this".

Any suggestions would be greatly appreciated. If more information is required just let me know and I'll provide it. Thanks!

--Mike