pptp error 721 GRE problem

Hi,

I have a intermittent problem with windows 2000 and 98 client establishing a
pptp connection to a 2000 server. this was not a problem until sp4 for 2000.

My network involves

NET
v
netgear router RG114
v
netgear router RG114 > win2k client
v
firewall
v
2k server

NOTE:
- From the NET a 2k client and a 98 machine can sometimes establish a pptp
connection.
- When they cant i get error 721 on 2000 and 650 on 98.
- From the win2k client on the middle router i can always establish a PPTP
connection.
- From an XP clinet on my LAN i can always establish a PPTP connection.
- The errors displayed are related to GRE (protocol 47) but it was working
before SP4.
- When attempting a connection my firewall reports PPTP only when it fails.
- But Firewall reports PPTP and GRE when it succeeds.
- Router must support GRE as microsoft support say connection would never
have worked.
- Firewall set to report MATCH and NO MATCH. when failed no GRE mismatch
reported.
- Hence I think GRE does not seem to get this far.

TESTS:
- 2k dial up adapter set to PPTP only. ie so time not wasted with timeout on
unwated protocols.
- amending 2000 server registry as per kb article
http://support.microsoft.com/?id=271731 and
http://support.microsoft.com/?id=810839 . Set an no address validate.
- rolling back to SP3.
- making sure no L2PT ports are avaible from available on 2000 server.

Im at a loss to explain why sometimes i can connect and other times i get
the errors above. Can someone please help ?

Thanks for any information at all
Scott.