Airport connection problems between Leopard and Tiger

Hi all,

I have the following problem with Airport Internet Sharing:

When I enable Airport Internet Sharing on my iMac with Leopard (10.5.3) and want to use it with a Macbook running Tiger, one of the following things will happen:

1. A connection is made, but no IP address is received by my MacBook, so the Macbook is left with a self assigned IP address with no internet connection (of course). This was the case before the latest update of Leopard. Sometimes, it worked without a problem, but after a while the DHCP refreshed and left the MacBook without an IP address, and using the renew button in the Network Preferences did not seem to accomplish anything.

2. A connection is made, an IP address is received but traffic is extremely slow. This behaviour seems to be new since the latest Leopard update.
In the log files of the iMac the following messages appeared:

natd\[2454\]: failed to write packet back (No route to host)
natd\[2454\]: failed to write packet back (Host is down)

Firewall is turned off on the macbook and set to "allow all incoming connections" on the iMac.

Anyone an idea how to solve this? I also tried using a different Macbook with Tiger and got the same results.
Moreover, when I use an old G4 AGP Graphics machine running Tiger to share the internet connection, it works flawlessly.

Thanks in advance!