Does the PS3 have a static IP? If so, you might try letting the router assign the IP to the PS3. Also make sure uPNP is turned on in the router. It's located under the advanced tab -->> advanced network menu.
Sometimes you can run into problems when the device in this case your PS3 had a static IP that was manually entered into the console. If you still want to give it a static ip, set the console to automatic, then use the DHCP reservation list on the router to lock it to one ip.
DHCP reservation is under the basic tab -->> Network settings
Another thing to check is see if dynamic fragmentation is checked. I had to turn mine off to eliminate some disconnects on from Xbox live. That option is under gamefuel. If after doing all that you are still experiencing problems, you might try adding a gaming rule to open some ports. www.portforward.com is a good site to find out what ports you might need to open and how to do it. BTW, the 4500 comes pre-programed with some gaming rules.
Give that a try and post back.