VNC Viewer (listen) crashes on connection of SC (WAN ONLY)

[some1]

Hello!
I got an odd problem!
Ok - the setup

Windows XP Pro Sp1 (on all the computers)
Linksys WAG54G (ports 5500- 6000 open to the correct PC)
dynamicIP - with Dyndns domain
Lastest VNC (VltraVncSC based on RC23)
ZoneAlarm - tho does't matter if I disable it
WindowsFirewall - tho doesn't matter if I disable it

I can run SC, and the viewer on my LAN, using INternalIPs (192.168.xx.xx) (laptop to PC) thats all fine
I can run SC, and the viewer on my LAN, using EXternalIP (88.106.xx.xx tho it chances every 24 hours) (laptop to PC) thats all fine
I can run SC, and the viewer on my LAN, using Dyndns domain (www.xxxxxxxx.mine.nu)(laptop to PC) thats all fine

*note, ive xxxx out the personal details*

But when I go and get a friend to run it, hes start "connection made", and my viewer crashers! (it just freezes, I cant right click on it!) i have to ctrl Alt and delete it!

Ive tired using just my external IP, dyndns domain, both with and without encrytion! but it mine always crashes!

any ideas?