vnc access from external network

hello i’m working on a vine server v3.0 installed on mac os x tiger 10.4.11. The computer is a powermac g4. It works wonderfully on my lan, but it doesn’t always work over the wan. I mean, sometimes the interface shows green external address (so it should work from outside) but sometimes it doesn’t even appear (LAN ip is always green). Does it mean that the desktop server is not working properly?
Here’s my setup:

server lan address
system server listening on port 5900
‘desktop’ server listening on port 6893
no SSL (not for now)
Both of them work on LAN. I configured my netgear router to forward only port 6893 to the server. Therefore, system server is not available from the outside. Concerning Tiger firewall (g4’s one of course), i’ve opened both 5900 and 6893. ‘Apple remote desktop’ service in services tab is disabled. I checked out every port setting using network utility, everything is alright…why do i can’t access from the outside?