I think the issue is with your broadband router/cable modem, as I get the same issue.
I have 2 broadband routers, as Netopia and a Zyxel. I've used the Netopia for years, its old but works fine. When I'm in game, the server browser works as normal.
The Zyxel is a much newer router when I'm in game, the server browser shows up most servers with really high pings. Its the way the server browser "floods" the network that causes my router to think its a denial of service (dos). If I look at my router page while RO2 server browser is refreshing, the cpu of my router is 100%.
Try setting up a few servers as favourites, when server browser refreshes, go to Favourites. If you refresh in here, and you only have a few servers, then they should appear to have normal pings.
You may need to change the Max NAT/Firewall Session Per User on your router, it may be called something else, but it will be under NAT on your router.