The following things will cause you to spike and thusly ruin everyones game:
1) Any sustained upload of any kind from ANY COMPUTER ATTACHED TO YOUR INTERNET CONNECTION that is not throttled through a complex router QoS (Quality of Service) setup. Torrents are the largest offender.
2) Large sustained downloads on slow connections (like 700k dsl). Windows automatic updates or other programs auto updates are probably the most common but roomates watching movies on hulu or similar is a growing issue.
2) Poor wireless reception, like stealing the neighbors connection instead of paying for your own, trying to shoot the signal through 6 feet of concrete or if you live in a highly congested area and multiple people are using the same channel and heavily at that. For gaming always use a cord.
3) Your isp has a broken router someplace or your isp is awful at bandwidth management and oversold their connection a ridiculous amount. A truely ridiculous amount. In all my years I've only seen this happen a handful of times and they usually fix it within an hour.
There is no easy way to fix these issues besides simply not using your internet. QoS stuff is complicated and installing netlimiter on each PC and then manually limiting every offending program is time consuming and you would have to know which programs to limit to begin with.
If you are feeling tech adventerous I would recommend flashing your router with DD-WRT or similar (Tomato) firmware and tooling around with QoS. You may obliterate your router though.
