Hey Dragon,
First and foremost, "Blocked outgoing ICMP packet (ICMP type 3)" is normal to appear in router logs. So an ICMP packet with a message type 3 (Destination Unreachable) lets you know that the machine you tried to reach may not be listening on the port used (if the port is not firewalled. If it is, then what happens depends on the config of your own firewall). So if you can, determine which port your ilink uses and you can setup port forwarding just to make sure that's not an issue. Or put an exception for that port in the firewall inside the router.
Secondly, Well, usually video over the internet does take a few seconds to buffer. This does cause the initial buffering distortion and the occasional buffering delay while in the middle of it. How to avoid this issue? You could setup QoS inside the DGL-4500 for your ilink product to receive the best possible bandwidth allocation. Just login to the router, click on advanced on the top, then choose either WISH or GAMEFUEL on the left hand side. Under the WISH option you could setup a priority and classify it as Video (VI) and that may give you better performance. You would just need to know the IP address that is given to your Bravia iLink.
Best of Luck,
Russell