First off quit playing wireless and downloading porn at the same time. Go to Home Depot or Best Buy and get an actual network cable the length that you will need to run that cable from your PC to your router. I used to have that problem when I was playing in a wireless network.
This also happens when other people are using your network like streaming video or downloading, even playing another game.
The next thing you can do is to go into your router and port forward all the ports for UDK and Steam. Also close out all steam windows. It will still run in your toolbar but it will not be streaming ads.
You can also get Razor’s Game Booster. This will shut down any extra programs running on your computer that are just eating up processing power and any programs possibly accessing the internet.
Run your antivirus/spyware software and make sure you PC is clean and also not accessing the internet.
You can also set QOS or Quality of Service on your router basically telling the router to use a certain percentage of it’s bandwidth for your game and computer. There are instructions on the internet for your specific router. Just follow them.
Lastly, if you share internet, you will always risk lag spikes with others are on your network. If you have no choice but to use wireless, make sure that there are no unauthorized people on your wireless network. Sometimes you would be surprised if someone is leeching off of you.