First:
In a default setup you should also open / forward port 4665 UDP to your amule box. It is not absolutely necessary for amule to work but it should improve your sources searching.
Second:
I strongly recommend you to update to amule 2.0.1, rc7 is lacking several features which are now, if I recall correctly, mandatory in order to connect to the big servers (Razorback2 i.e.).
And about the problem you asked:
I've seen such behaviour back when I used a cheapo D-Link router. It started, amule connected, there were several hundreds of connections incoming and the router started hallucinating there was some kind of attack, flooded its log about "ping of death" attacks and discarded packages. Because of that I had often problems achieving a HighID, just because the server tried to reach my port 4662, the package was dropped by the router and the server thought (with a reason) that I was firewalled...
