First question I should ask is what signal strength is the PS3 getting from the router? If it's low enough it will cause disconnects.
If it is still doing this when using a wired connection, though, you may want to go into the router settings on your computer and open certain ports- this could also be the cause. The ports you want are:
TCP Ports: 80, 443, 5223, and 10070 - 10080
UDP Ports: 3478, 3479, 3658, and 10070