I think you mean "WPA2" not "WAP2", but the built in Skype Client works for me over a WPA2 connection. No connection losses so far.

If your router has UPNP you could try to disable this. Skype usually works well without this, but tries to forward it's ports over it (not sure about the N900 implementation, but at least the windows one does). Maybe there is some kind of bug with UPNP in your router. Well..it should be disabled anyways as it can be a real security problem sometimes (e.g. malicious application forwarding Windows Filesharing Ports to the internet).