Changing Ethernet cable has been mentioned in previous posts - couldn't see the point in that as it either works or it doesn't - but when my PS3 lost it"s connection, that's what I did and that sorted out the problem. So, that might be worth trying, also, the filter box might be playing up and renewing that could help (I don't know how long these boxes last).
This approach all starts costing money, not a lot, but unless you've got a mate who's system's working fine and will let you try out your components in their set-up it's difficult to isolate the problem.